selftests: forwarding: mirror_gre_vlan_bridge_1q: Fix tunnel name

The "ip6gretap" in the test name refers to the tunnel device type that
the test is supposed to be testing. However test_ip6gretap_forbidden()
tests, due to a typo, a gretap tunnel. Fix the typo.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>