net: bridge: remove fdb_notify forward declaration

fdb_notify() has a forward declaration because its first caller,
fdb_delete(), is declared before 3 functions that fdb_notify() needs:
fdb_to_nud(), fdb_fill_info() and fdb_nlmsg_size().

This patch moves the aforementioned 4 functions above fdb_delete() and
deletes the forward declaration.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>