Use shared socket/net lock for IP sockets

Use shared socket/net lock for IP sockets

so{,un}lock_shared() take the shared net lock for PF_INET and PF_INET6
while sticking to the exclusive rwlock elsewhere.

getsockopt(2), getsockname(2) and getpeername(2) (all UNLOCK) do not
write, so the exclusive net lock is overkill here.

OK mvs