vxlan: Use linked list to traverse FDB entries

In preparation for removing the fixed size hash table, convert FDB entry
traversal to use the newly added FDB linked list.

No functional changes intended.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250415121143.345227-9-idosch@nvidia.com


Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>