vxlan: changelink: Postpone vxlan_config_apply()

When an FDB entry is vetoed, it is necessary to unroll the changes that
have already been done. To avoid having to unroll vxlan_config_apply(),
postpone the call after the point where the vetoing takes place. Since
the call can't fail, it doesn't necessitate any cleanups in the
preceding FDB update logic.

Correspondingly, move down the mod_timer() call as well.

References to *dst need to be replaced with references to conf.
Additionally, old_dst and old_age_interval are not necessary anymore,
and therefore drop them.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>