io_uring: introduce IO_REQ_LINK_FLAGS

Add a macro for all link request flags to avoid duplication.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/df38b883e31e7e0ca4e364d25a0743862961b180.1650056133.git.asml.silence@gmail.com


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