mlxsw: spectrum: Add packet traps for BFD packets

Bidirectional Forwarding Detection (BFD) provides "low-overhead,
short-duration detection of failures in the path between adjacent
forwarding engines" (RFC 5880).

This is accomplished by exchanging BFD packets between the two
forwarding engines. Up until now these packets were trapped via the
general local delivery (i.e., IP2ME) trap which also traps a lot of
other packets that are not as time-sensitive as BFD packets.

Expose dedicated traps for BFD packets so that user space could
configure a dedicated policer for them.

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