block: use plug request list tail for one-shot backmerge attempt

Previously, the block layer stored the requests in the plug list in
LIFO order. For this reason, blk_attempt_plug_merge() would check
just the head entry for a back merge attempt, and abort after that
unless requests for multiple queues existed in the plug list. If more
than one request is present in the plug list, this makes the one-shot
back merging less useful than before, as it'll always fail to find a
quick merge candidate.

Use the tail entry for the one-shot merge attempt, which is the last
added request in the list. If that fails, abort immediately unless
there are multiple queues available. If multiple queues are available,
then scan the list. Ideally the latter scan would be a backwards scan
of the list, but as it currently stands, the plug list is singly linked
and hence this isn't easily feasible.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/linux-block/20250611121626.7252-1-abuehaze@amazon.com/


Reported-by: Hazem Mohamed Abuelfotoh <abuehaze@amazon.com>
Fixes: e70c301f ("block: don't reorder requests in blk_add_rq_to_plug")
Signed-off-by: Jens Axboe <axboe@kernel.dk>