SOCK_SENDBUF_MTX
mtx_lock(SOCK_SENDBUF_MTX(so))
mtx_unlock(SOCK_SENDBUF_MTX(so))
mtx_assert(SOCK_SENDBUF_MTX(so), MA_OWNED)
mtx_assert(SOCK_SENDBUF_MTX(so), MA_NOTOWNED)
return (which == SO_RCV ? SOCK_RECVBUF_MTX(so) : SOCK_SENDBUF_MTX(so));