staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5858
Reviewed-on: http://review.whamcloud.com/12554


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>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>