vxlan: Unmark offloaded bit on replaced FDB entries

When rdst of an offloaded FDB entry is replaced, it certainly isn't
offloaded anymore. Drivers are notified about such replacements, and can
re-mark the entry as offloaded again if they so wish. However until a
driver does so explicitly, assume a replaced FDB entry is not offloaded.

Note that replaces coming via vxlan_fdb_external_learn_add() are always
immediately followed by an explicit offload marking.

Fixes: 0efe1173 ("vxlan: Support marking RDSTs as offloaded")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>