bridge: Add per-VLAN netlink handling for neigh_forward_grat

bridge: Add per-VLAN netlink handling for neigh_forward_grat

Add netlink handlers for the per-VLAN neigh_forward_grat option via
BRIDGE_VLANDB_ENTRY_NEIGH_FORWARD_GRAT attribute.

The per-VLAN option provides fine-grained control, allowing different
VLANs on the same port to have different gratuitous ARP/unsolicited NA
forwarding behavior.

This enables control via 'bridge' commands:
 # bridge vlan set dev eth0 vid 10 neigh_suppress on
 # bridge vlan set dev eth0 vid 10 neigh_forward_grat on

Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260511065936.4173106-6-danieller@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>