vxlan: Don't override user-added entries with ext-learned ones

When an external learning event collides with an user-added entry, the
user-added entry shouldn't be taken over. Otherwise on an unlearn event
the entry would be completely lost, even though the user added it by
hand.

Therefore skip update of FDB flags and state for these cases. This is in
accordance with the bridge behavior.

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