block: fix get_max_io_size()

A previous commit aligning splits to physical block sizes inadvertently
modified one return case such that that it now returns 0 length splits
when the number of sectors doesn't exceed the physical offset. This
later hits a BUG in bio_split(). Restore the previous working behavior.

Fixes: 9cc5169cd4 ("block: Improve physical block alignment of split bios")
Reported-by: Eric Deal <eric.deal@wdc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-merge.c

@@ -154,7 +154,7 @@ static inline unsigned get_max_io_size(struct request_queue *q,
 	if (max_sectors > start_offset)
 		return max_sectors - start_offset;
 
-	return sectors & (lbs - 1);
+	return sectors & ~(lbs - 1);
 }
 
 static inline unsigned get_max_segment_size(const struct request_queue *q,