net: bridge: remove fdb_insert forward declaration

fdb_insert() has a forward declaration because its first caller,
br_fdb_changeaddr(), is declared before fdb_create(), a function which
fdb_insert() needs.

This patch moves the 2 functions above br_fdb_changeaddr() and deletes
the forward declaration for fdb_insert().

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>