net: ip6_gre: fix moving ip6gre between namespaces

Support for moving IPv4 GRE tunnels between namespaces was added in
commit b57708ad ("gre: add x-netns support"). The respective change
for IPv6 tunnels, commit 22f08069 ("ip6gre: add x-netns support")
did not drop NETIF_F_NETNS_LOCAL flag so moving them from one netns to
another is still denied in IPv6 case. Drop NETIF_F_NETNS_LOCAL flag from
ip6gre tunnels to allow moving ip6gre tunnel endpoints between network
namespaces.

Signed-off-by: Niko Kortstrom <niko.kortstrom@nokia.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>