block: return the correct bvec when checking for gaps

After commit 07173c3ec276 ("block: enable multipage bvecs"), a bvec can
have multiple pages. But bio_will_gap() still assumes one page bvec while
checking for merging. If the pages in the bvec go across the
seg_boundary_mask, this check for merging can potentially succeed if only
the 1st page is tested, and can fail if all the pages are tested.

Later, when SCSI builds the SG list the same check for merging is done in
__blk_segment_map_sg_merge() with all the pages in the bvec tested. This
time the check may fail if the pages in bvec go across the
seg_boundary_mask (but tested okay in bio_will_gap() earlier, so those
BIOs were merged). If this check fails, we end up with a broken SG list
for drivers assuming the SG list not having offsets in intermediate pages.
This results in incorrect pages written to the disk.

Fix this by returning the multi-page bvec when testing gaps for merging.

Cc: Jens Axboe <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Cc: Pavel Begunkov <[email protected]>
Cc: Ming Lei <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Jeffle Xu <[email protected]>
Cc: [email protected]
Cc: [email protected]
Fixes: 07173c3ec276 ("block: enable multipage bvecs")
Signed-off-by: Long Li <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

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