block: check morerequests for multiple_queues in blk_attempt_plug_merge

RAID arrays check/repair operations benefit a lot from merging requests.
If we only check the previous entry for merge attempt, many merge will be
missed. As a result, significant regression is observed for RAID check
and repair.

Fix this by checking more than just the previous entry when
plug->multiple_queues == true.

Fixes: d38a9c04c0d5 ("block: only check previous entry for plug merge attempt")
Cc: stable@vger.kernel.org # v5.16
Reported-by: Larkin Lowrey <llowrey@nuclearwinter.com>
Reported-by: Wilson Jonathan <i400sjon@gmail.com>
Reported-by: Roger Heflin <rogerheflin@gmail.com>
Signed-off-by: Song Liu <song@kernel.org>