io_uring/rw: clean up mshot forced sync mode

Move code forcing synchronous execution of multishot read requests out
a more generic __io_read().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/4ad7b928c776d1ad59addb9fff64ef2d1fc474d5.1739919038.git.asml.silence@gmail.com


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