LU-1603 ofd: Fix null pointer dereferences in ldiskfs_statfs()

On an OSS running the OFD-OSD stack, unmounting all OSTs makes all OFD
and OSD keys (i.e., lu_context_key) quiescent.  Next time the OSTs are
remounted, ofd_stack_init() will try to refill the quiescent osd_key
into the context, resulting in a null pointer dereference in
ldiskfs_statfs().  Before the problem motivating LCT_QUIESCENT and
LCT_REMEMBER can be revisited, the solution is to refill contexts
after new device types have been "revived" via allocations of their
first device instances.

Change-Id: I4835afb88fae460c21501df95765afd29e570860
Signed-off-by: Li Wei <liwei@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/3353
Tested-by: Hudson
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>