mlxsw: spectrum: Treat IPv6 link-local SIP as an exception

IPv6 packets that need to be forwarded and have a link-local source IP are
dropped by the kernel and an ICMPv6 "Destination unreachable" is sent to
the sending host.

As such, change the trap group of such packets so that they do not
interfere with IPv6 management packets. In the future this trap will be
exposed as an exception via devlink-trap.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>