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