geneve: allow to clear ttl inherit

As Michal remaind, we should allow to clear ttl inherit. Then we will
have three states:

1. set the flag, and do ttl inherit.
2. do not set the flag, use configured ttl value, or default ttl (0) if
   not set.
3. disable ttl inherit, use previous configured ttl value, or default ttl (0).

Fixes: 52d0d404

 ("geneve: add ttl inherit support")
CC: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>