ktls: Fix interlocking between ktls_enable_rx() and listen(2)

The TCP_TXTLS_ENABLE and TCP_RXTLS_ENABLE socket option handlers check
whether the socket is listening socket and fail if so, but this check is
racy. Since we have to lock the socket buffer later anyway, defer the
check to that point.

ktls_enable_tx() locks the send buffer's I/O lock, which will fail if
the socket is a listening socket, so no explicit checks are needed. In
ktls_enable_rx(), which does not acquire the I/O lock (see the review
for some discussion on this), use an explicit SOLISTENING() check after
locking the recv socket buffer.

Otherwise, a concurrent solisten_proto() call can trigger crashes and
memory leaks by wiping out socket buffers as ktls_enable_*() is
modifying them.

Also make sure that a KTLS-enabled socket can't be converted to a
listening socket, and use SOCK_(SEND|RECV)BUF_LOCK macros instead of the
old ones while here.

Add some simple regression tests involving listen(2).

Reported by: syzkaller
MFC after: 2 weeks
Reviewed by: gallatin, glebius, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38504