blk-merge: fix computing bio->bi_seg_front_size in case of single segment
blk-merge: fix computing bio->bi_seg_front_size in case of single segment

When bio has only one physical segment, we should set bio's
bi_seg_front_size as the real(final) size of the single segment.

Fixes: 02e707424c2ea(blk-merge: fix blk_bio_segment_split)
Reported-by: Markus Trippelsdorf <300a2d193222dd0c23d880e16d727d352b404a01@trippelsdorf.de>
Tested-by: Markus Trippelsdorf <300a2d193222dd0c23d880e16d727d352b404a01@trippelsdorf.de>
Signed-off-by: Ming Lei <8d0fc11caeb4659419f0ba3104eb4a9d30848d42@canonical.com>
Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@fb.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs