io_uring: refactor io_get_sequence()

Clean up io_get_sequence() and add a comment describing the magic around
sequence correction.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f55dc409936b8afa4698d24b8677a34d31077ccb.1623949695.git.asml.silence@gmail.com


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