[roll] Roll fuchsia [dhcpd] Fix src/dst address setting in replies

Fixes two separate bugs introduced by https://fxrev.dev/699943:

- https://fxbug.dev/105371, in which we mistakenly set the `src_ip` to
  match the sender's `src_ip`. The fix is to get the `src_ip` from the
  `ServerIdentifier` option, panic-ing if it is not present.
- https://fxbug.dev/105389, in which we set the DHCP Offer as broadcast
  even when the Discover's Broadcast bit is 0. The fix is to use the
  already-computed `dst` addr, which takes this into account.

Test: network-conformance-testing
Original-Fixed: 105371,105389
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/705108
Original-Revision: 1fddb148df84224f71f799778b7b41ec3aa2a03c
GitOrigin-RevId: ed558ce5dabb77070b78c247456ccaf65c11b9d6
Change-Id: Id63a2e796855e8ef7a3d0e6d27f84f83f876fbe8

stem

diff --git a/stem b/stem
index 7ad6602..a8e274a 100644
--- a/stem
+++ b/stem
@@ -10,7 +10,7 @@
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-             revision="0c5eddbd2bffe410dfc17cb24ec33ca0e51bfbb9"/>
+             revision="1fddb148df84224f71f799778b7b41ec3aa2a03c"/>
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