net: replace sockbuf::sb_rwlock with a waitqueue

sb_rwlock is used to serialize concurrent writers (or readers) to the same
socket buffer, but is quite expensive as it requires 4 atomic operations
per transaction, even if there is no contention.

Replace it with a waitqueue, and use the sockbuf::sb_mtx for serialization.
This still has exactly the same cost, but we can later move sblock() and
sbunlock() into contexts where the sockbuf::sb_mtx is already acquired.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>