Since commit c67f3b8b78e50c6df7c057d6cf108e4d6b4312d0 the sockbuf
mutexes belong to the containing socket. Sockbufs contain a pointer to
a mutex, which by default is initialized to the corresponding mutexes in
the socket. The SOCKBUF_LOCK() etc. macros operate on this pointer.
However, the pointer is clobbered by listen(2) so it's not safe to use
them unless one is sure that the socket is not a listening socket.

This change introduces a new set of macros which lock socket buffers
through the socket. This is a bit cheaper since it removes the pointer
indirection, and allows one to safely lock socket buffers and then check
for a listening socket.

I have some changes staged which convert most consumers to use the new
macros, though this introduces a fair bit of churn. The new locking
macros can be trivially implemented in terms of the existing ones on
stable/13, though, so I don't think it's a major problem to do the sweep
and MFC.