io_uring: rename sr->msg into umsg

Every second field in send/recv is called msg, make it a bit more
understandable by renaming ->msg, which is a user provided ptr,
to ->umsg.

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