LU-12685 llite: fix check for mem-alloc failure.

LU-12685 llite: fix check for mem-alloc failure.

A change to allocation of op_data in ll_statahead_thread()
appears to have assumed that OBD_ALLOC_PTR() would set the
pointer to ERR_PTR(-ENOMEM) on failure.  It actually sets
it to NULL, so the test needs to be changed accordingly.

Test-Parameters: trivial
Fixes: ae828cd3b092 ("LU-4684 llite: add lock for dir layout data")
Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: I37521d2da50d71ed6fa0f9f05b7cfb848f0d47d9
Reviewed-on: https://review.whamcloud.com/35873
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>