LU-5858 obdclass: eliminate NULL error return

LU-5858 obdclass: eliminate NULL error return

Always return an ERR_PTR() on errors, never return a NULL,
in lu_object_find_slice().  Also clean up callers who
no longer need special case handling of NULL returns.

Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Change-Id: I57ddb38abaec7caf57bb63a75dbd76e181ba72b2
Reviewed-on: http://review.whamcloud.com/12554
Tested-by: Jenkins
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>