mctp: Allow local delivery to the null EID

[ Upstream commit 1f6c77ac ]

We may need to receive packets addressed to the null EID (==0), but
addressed to us at the physical layer.

This change adds a lookup for local routes when we see a packet
addressed to EID 0, and a local phys address.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 5093bbfc ("mctp: perform route lookups under a RCU read-side lock")
Signed-off-by: Sasha Levin <sashal@kernel.org>