geneve: add ttl inherit support

Similar with commit 72f6d71e ("vxlan: add ttl inherit support"),
currently ttl == 0 means "use whatever default value" on geneve instead
of inherit inner ttl. To respect compatibility with old behavior, let's
add a new IFLA_GENEVE_TTL_INHERIT for geneve ttl inherit support.

Reported-by: Jianlin Shi <jishi@redhat.com>
Suggested-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>