alloc_ordered_workqueue may fail and return NULL.  The fix captures the
failure and handles it properly to avoid potential NULL pointer
dereferences.
Signed-off-by: NKangjie Lu <kjlu@umn.edu>
Reviewed-by: NShiraz, Saleem <shiraz.saleem@intel.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>