performance/quick-read: fix race condition in unlink

use same lock (inode->lock), while incrementing/decrementing local->open_count.

Change-Id: I08cbab5b5dec09b6057f43324fe3152f1564ce46
BUG: 902174
Signed-off-by: Raghavendra G <raghavendra@gluster.com>
Reviewed-on: http://review.gluster.org/4396
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>