LU-3884 lfsck: LFSCK should NOT hold root object

LU-3884 lfsck: LFSCK should NOT hold root object

The root object may be accessed via local storage stack and release
the reference by lu_object_put_nocache(), then the root object will
be marked as LU_OBJECT_HEARD_BANSHEE. On the other hand, the LFSCK
still holds reference on the root object. Under such case, if some
others want to access the root object, they have to wait the LFSCK
to release the root object and re-generate the root object in RAM.
But if the LFSCK does not release the reference on the root object
until the device umount, then all the others will hung there.

Test-Parameters: testlist=sanity-lfsck
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I068c442ae808bb2f018e93e0a584a35dcf4c2f9c
Reviewed-on: http://review.whamcloud.com/7643
Tested-by: Hudson
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>