block: reorder the REQ_ flags

Keep the op-specific flag last so that they are clearly separate from
the generic flags.  Various recent commits just kept adding new flags
at the end.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220512061408.1826595-1-hch@lst.de


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