The locking annotations for struct sockbuf originally used the key from

The locking annotations for struct sockbuf originally used the key from
struct socket.  When sockbuf.h was moved out of socketvar.h, the locking
key was no longer nearby.  Instead, add a new key for sockbuf and use
a single item for the socket buffer lock instead of separate entries for
receive vs send buffers.

Reviewed by:	adrian
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D4901