Mark `so_rcv' sockbuf of udp(4) sockets as SB_OWNLOCK.

Mark `so_rcv' sockbuf of udp(4) sockets as SB_OWNLOCK.

sbappend*() and soreceive() of SB_MTXLOCK marked sockets uses `sb_mtx'
mutex(9) for protection, meanwhile buffer usage check and corresponding
sbwait() sleep still serialized by solock(). Mark udp(4) as SB_OWNLOCK
to avoid solock() serialization and rely to `sb_mtx' mutex(9). The
`sb_state' and `sb_flags' modifications must be protected by `sb_mtx'
too.

ok bluhm