io_uring: don't take fs for recvmsg/sendmsg

We don't even allow not plain data msg_control, which is disallowed in
__sys_{send,revb}msg_sock(). So no need in fs for IORING_OP_SENDMSG and
IORING_OP_RECVMSG. fs->lock is less contanged not as much as before, but
there are cases that can be, e.g. IOSQE_ASYNC.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>