IB/core: Free umem when mm is already gone

Free umem when task's mm is already destroyed by the time
ib_umem_release gets called.

Found by Dotan Barak at Mellanox.

Signed-off-by: Eli Cohen <[e-mail]>
Signed-off-by: Roland Dreier <[e-mail]>

drivers/infiniband/core/umem.c

···
 	__ib_umem_release(umem->context->device, umem, 1);
 
 	mm = get_task_mm(current);
-	if (!mm)
+	if (!mm) {
+		kfree(umem);
 		return;
+	}
 
 	diff = PAGE_ALIGN(umem->length + umem->offset) >> PAGE_SHIFT;