io_uring: extra a helper for drain init

Add a helper io_init_req_drain for initialising requests with
IOSQE_DRAIN set. Also move bits from preambule of io_drain_req() in
there, because we already modify all the bits needed inside the helper.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/dcb412825b35b1cb8891245a387d7d69f8d14cef.1633107393.git.asml.silence@gmail.com


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