geneve: correctly handle ipv6.disable module parameter

When IPv6 is compiled but disabled at runtime, geneve_sock_add returns
-EAFNOSUPPORT. For metadata based tunnels, this causes failure of the whole
operation of bringing up the tunnel.

Ignore failure of IPv6 socket creation for metadata based tunnels caused by
IPv6 not being available.

This is the same fix as what commit d074bf96 ("vxlan: correctly handle
ipv6.disable module parameter") is doing for vxlan.

Note there's also commit c0a47e44

 ("geneve: should not call rt6_lookup()
when ipv6 was disabled") which fixes a similar issue but for regular
tunnels, while this patch is needed for metadata based tunnels.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>