getsockopt: improve locking for SOL_SOCKET level socket options

Ensure SOLISTENING() is done inside SOCK_LOCK()/SOCK_UNLOCK()
for getsockopt() handling of SOL_SOCKET-level socket options.

Reviewed by: markj, rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46881

(cherry picked from commit 3326ab87cc22cb70a41e13aefd8684ff0feed01f)