raid5]
  8,16   0     6639     2.453620460  2579  G   W 7493144 + 8 [md0_raid5]
  8,16   0     6640     2.453639311  2579  Q   W 7493120 + 8 [md0_raid5]
  8,16   0     6641     2.453639842  2579  G   W 7493120 + 8 [md0_raid5]
The bio(7493144) first and bio(7493120) later.So the subsequent
bios will be divided into two parts.
When flushing plug-list,because elv_attempt_insert_merge only support
backmerge,not supporting frontmerge.
So rq[7493120 + 24] can't merge with rq[7493144 + 104].

From my test,i found those situation can count 25% in our system.
Using this patch, there is no this situation.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
CC:Shaohua Li <shli@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
8Ó