vxlan: Convert FDB flushing to RCU

Instead of holding the FDB hash lock when traversing the FDB linked list
during flushing, use RCU and only acquire the lock for entries that need
to be flushed.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250415121143.345227-11-idosch@nvidia.com


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