[dns/sntp-client] use Instance as a constructor parameter (#5253)

diff --git a/src/core/net/dns_client.cpp b/src/core/net/dns_client.cpp
index 0919d62..0839623 100644
--- a/src/core/net/dns_client.cpp
+++ b/src/core/net/dns_client.cpp
@@ -72,10 +72,10 @@
     mDestinationAddress.Clear();
 }
 
-Client::Client(Ip6::Netif &aNetif)
-    : mSocket(aNetif.GetInstance())
+Client::Client(Instance &aInstance)
+    : mSocket(aInstance)
     , mMessageId(0)
-    , mRetransmissionTimer(aNetif.GetInstance(), Client::HandleRetransmissionTimer, this)
+    , mRetransmissionTimer(aInstance, Client::HandleRetransmissionTimer, this)
 {
 }
 
diff --git a/src/core/net/dns_client.hpp b/src/core/net/dns_client.hpp
index 3cd8f89..32d38a4 100644
--- a/src/core/net/dns_client.hpp
+++ b/src/core/net/dns_client.hpp
@@ -129,10 +129,10 @@
     /**
      * This constructor initializes the object.
      *
-     * @param[in]  aNetif    A reference to the network interface that DNS client should be assigned to.
+     * @param[in]  aInstance     A reference to the OpenThread instance.
      *
      */
-    explicit Client(Ip6::Netif &aNetif);
+    explicit Client(Instance &aInstance);
 
     /**
      * This method starts the DNS client.
diff --git a/src/core/net/sntp_client.cpp b/src/core/net/sntp_client.cpp
index a8bc0e4..bfcc59e 100644
--- a/src/core/net/sntp_client.cpp
+++ b/src/core/net/sntp_client.cpp
@@ -89,9 +89,9 @@
     mDestinationAddress.Clear();
 }
 
-Client::Client(Ip6::Netif &aNetif)
-    : mSocket(aNetif.GetInstance())
-    , mRetransmissionTimer(aNetif.GetInstance(), Client::HandleRetransmissionTimer, this)
+Client::Client(Instance &aInstance)
+    : mSocket(aInstance)
+    , mRetransmissionTimer(aInstance, Client::HandleRetransmissionTimer, this)
     , mUnixEra(0)
 {
 }
diff --git a/src/core/net/sntp_client.hpp b/src/core/net/sntp_client.hpp
index 722b57f..2e35918 100644
--- a/src/core/net/sntp_client.hpp
+++ b/src/core/net/sntp_client.hpp
@@ -493,10 +493,10 @@
     /**
      * This constructor initializes the object.
      *
-     * @param[in]  aNetif    A reference to the network interface that SNTP client should be assigned to.
+     * @param[in]  aInstance     A reference to the OpenThread instance.
      *
      */
-    explicit Client(Ip6::Netif &aNetif);
+    explicit Client(Instance &aInstance);
 
     /**
      * This method starts the SNTP client.
diff --git a/src/core/thread/thread_netif.cpp b/src/core/thread/thread_netif.cpp
index 2965d4d..1bb64ac 100644
--- a/src/core/thread/thread_netif.cpp
+++ b/src/core/thread/thread_netif.cpp
@@ -52,19 +52,19 @@
     , mCoap(aInstance)
 #if OPENTHREAD_CONFIG_DHCP6_CLIENT_ENABLE
     , mDhcp6Client(aInstance)
-#endif // OPENTHREAD_CONFIG_DHCP6_CLIENT_ENABLE
+#endif
 #if OPENTHREAD_CONFIG_DHCP6_SERVER_ENABLE
     , mDhcp6Server(aInstance)
-#endif // OPENTHREAD_CONFIG_DHCP6_SERVER_ENABLE
+#endif
 #if OPENTHREAD_CONFIG_IP6_SLAAC_ENABLE
     , mSlaac(aInstance)
 #endif
 #if OPENTHREAD_CONFIG_DNS_CLIENT_ENABLE
-    , mDnsClient(Get<ThreadNetif>())
-#endif // OPENTHREAD_CONFIG_DNS_CLIENT_ENABLE
+    , mDnsClient(aInstance)
+#endif
 #if OPENTHREAD_CONFIG_SNTP_CLIENT_ENABLE
-    , mSntpClient(Get<ThreadNetif>())
-#endif // OPENTHREAD_CONFIG_SNTP_CLIENT_ENABLE
+    , mSntpClient(aInstance)
+#endif
     , mActiveDataset(aInstance)
     , mPendingDataset(aInstance)
     , mKeyManager(aInstance)
@@ -89,21 +89,21 @@
 #endif
 #if OPENTHREAD_CONFIG_COMMISSIONER_ENABLE && OPENTHREAD_FTD
     , mCommissioner(aInstance)
-#endif // OPENTHREAD_CONFIG_COMMISSIONER_ENABLE && OPENTHREAD_FTD
+#endif
 #if OPENTHREAD_CONFIG_DTLS_ENABLE
     , mCoapSecure(aInstance)
 #endif
 #if OPENTHREAD_CONFIG_JOINER_ENABLE
     , mJoiner(aInstance)
-#endif // OPENTHREAD_CONFIG_JOINER_ENABLE
+#endif
 #if OPENTHREAD_CONFIG_JAM_DETECTION_ENABLE
     , mJamDetector(aInstance)
-#endif // OPENTHREAD_CONFIG_JAM_DETECTION_ENABLE
+#endif
 #if OPENTHREAD_FTD
     , mJoinerRouter(aInstance)
     , mLeader(aInstance)
     , mAddressResolver(aInstance)
-#endif // OPENTHREAD_FTD
+#endif
 #if (OPENTHREAD_CONFIG_THREAD_VERSION >= OT_THREAD_VERSION_1_2)
     , mBackboneRouterLeader(aInstance)
 #endif