io_uring/net: don't retry recvmsg() unnecessarily

If we're doing multishot receives, then we always end up doing two trips
through sock_recvmsg(). For protocols that sanely set msghdr->msg_inq,
then we don't need to waste time picking a new buffer and attempting a
new receive if there's nothing there.

Signed-off-by: Jens Axboe <axboe@kernel.dk>