gre: Always set TTL on outer packet to 64.

gre: Always set TTL on outer packet to 64.

Currently the TTL is copied from the inner packet of the tunnel to
the outer packet if the inner packet is IP.  This is good if your
GRE packets might make it into the input of your device but bad
if you want to be fully transparent.

This also resolves an inconsistency between tunnels set up using
the ioctl and using Netlink.  The ioctl version would force PMTUD
on if a fixed TTL is set as a backup way to prevent loops but it
never made it over to the newer Netlink code so obviously no one
cares too much about it.  This removes it to provide consistency
and transparency.

Basically, don't create loops and you will be happy.