test_objagg: Test the correct variable

There is a typo here.  We intended to check "objagg2" but we instead
test "objagg" which is not an error pointer.

Fixes: 9069a381 ("lib: objagg: implement optimization hints assembly and use hints for object creation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>