io_uring: indent left {send,recv}[msg]()

Flip over "if (sock)" condition with return on error, the upper layer
will take care. That change will be handy later, but already removes
an extra jump from hot path.

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