i40iw: Avoid writing to freed memory

iwpbl->iwmr points to the structure that contains iwpbl,
which is iwmr. Setting this to NULL would result in
writing to freed memory. So just free iwmr, and return.

Fixes: d3749841 ("i40iw: add files for iwarp interface")
Reported-by: Stefan Assmann <sassmann@redhat.com>
Signed-off-by: Mustafa Ismail <mustafa.ismail@intel.com>
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/hw/i40iw/i40iw_verbs.c

@@ -1925,8 +1925,7 @@ static int i40iw_dereg_mr(struct ib_mr *ib_mr)
 		}
 		if (iwpbl->pbl_allocated)
 			i40iw_free_pble(iwdev->pble_rsrc, palloc);
-		kfree(iwpbl->iwmr);
-		iwpbl->iwmr = NULL;
+		kfree(iwmr);
 		return 0;
 	}