net/netlabel: copy and paste bug in netlbl_cfg_unlbl_map_add()

This was copy and pasted from the IPv4 code.  We're calling the
ip4 version of that function and map4 is NULL.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>