[Netstack3] Add a comment to clarify locks in destroy_socket.

Added a comment that explains why TCP sockets have to be dropped
while holding socket lock.

Also updated the code to use cfg_if, which makes it easier to
understand.

Bug: 493359475
Change-Id: I2600271509ec259f31988bcf696ff5a4e960f368
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1547150
Reviewed-by: Bruno Dal Bo <brunodalbo@google.com>
Reviewed-by: Peter DeLong <pdelong@google.com>
Commit-Queue: Sergey Ulanov <sergeyu@google.com>