LU-6271 osc: faulty assertion in osc_object_prune()

LU-6271 osc: faulty assertion in osc_object_prune()

There may exist freeing pages in object's radix tree at
the time of osc_object_prune(), which causes failure at
the assertion of (osc->oo_npages == 0). This is a safe
race.

This problem is introduced in change at:
Lustre-commit: e8b421531c166b91ab5c1f417570c544bcdd050c
Lustre-change: http://review.whamcloud.com/16456

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I7d4e59bccfb012b870a2e8fa7ab99774def57349
Reviewed-on: http://review.whamcloud.com/16727
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>