socket: Move sockbuf mutexes into the owning socket

This is necessary to provide proper interlocking with listen(2), which
destroys the socket buffers. Otherwise, code must lock the socket
itself and check SOLISTENING(so), but most I/O paths do not otherwise
need to acquire the socket lock, so the extra overhead needed to check a
rare error case is undesirable.

listen(2) calls are relatively rare. Thus, the strategy is to have it
acquire all socket buffer locks when transitioning to a listening
socket. To do this safely, these locks must be stable, and not
destroyed during listen(2) as they are today. So, move them out of the
sockbuf and into the owning socket. For the sockbuf mutexes, keep a
pointer to the mutex in the sockbuf itself, for now. This can be
removed by replacing SOCKBUF_LOCK() etc. with macros which operate on
the socket itself, as was done for the sockbuf I/O locks.

Reviewed by: tuexen, gallatin
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31658