LU-1976 io: check index node before recycle empty leaf

LU-1976 io: check index node before recycle empty leaf

There may be multiple threads trying to shrink the same OI index
node to recycle different empty OI leaf nodes concurrently, and
someone may enlarged or split the index node before the threads
shrinking the index node to recycle the empty OI leaf nodes.
These concurrent OI index node modifications will change leaf
node position in the index node. So before shrinking the index
node, we need to check and adjust related frame pointer to avoid
to shrink the index node improperly as to crash the memory.

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Change-Id: I26063407f6c52f3e51ddb40e6c27aaa5cf63ec2b
Reviewed-on: http://review.whamcloud.com/4061
Tested-by: Hudson
Reviewed-by: wangdi <di.wang@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>