md:Add blk_plug in sync_thread.

Add blk_plug in sync_thread will increase the performance of sync.
Because sync_thread did not blk_plug,so when raid sync, the bio merge
not well.

Testing environment:
SATA controller: Intel Corporation 82801JI (ICH10 Family) SATA AHCI
Controller.
OS:Linux xxx 3.5.0-rc2+ #340 SMP Tue Jun 12 09:00:25 CST 2012
x86_64 x86_64 x86_64 GNU/Linux.
RAID5: four ST31000524NS disk.

Without blk_plug:recovery speed about 63M/Sec;
Add blk_plug:recovery speed about 120M/Sec.

Using blktrace:
blktrace -d /dev/sdb -w 60  -o -|blkparse -i -

without blk_plug:
Total (8,16):
 Reads Queued:      309811,     1239MiB	 Writes Queued:           0,        0KiB
 Read Dispatches:   283583,     1189MiB	 Write Dispatches:        0,        0KiB
 Reads Requeued:         0		 Writes Requeued:         0
 Reads Completed:   273351,     1149MiB	 Writes Completed:        0,        0KiB
 Read Merges:        23533,    94132KiB	 Write Merges:            0,        0KiB
 IO unplugs:             0        	 Timer unplugs:           0

add blk_plug:
Total (8,16):
 Reads Queued:      428697,     1714MiB	 Writes Queued:           0,        0KiB
 Read Dispatches:     3954,     1714MiB	 Write Dispatches:        0,        0KiB
 Reads Requeued:         0		 Writes Requeued:         0
 Reads Completed:     3956,     1715MiB	 Writes Completed:        0,        0KiB
 Read Merges:       424743,     1698MiB	 Write Merges:            0,        0KiB
 IO unplugs:             0        	 Timer unplugs:        3384

The ratio of merge will be markedly increased.

Signed-off-by: majianpeng <majianpeng@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>