mctp: Remove only static neighbour on RTM_DELNEIGH

Add neighbour source flag in mctp_neigh_remove(...) to allow removal of
only static neighbours.

This should be a no-op change and might be useful later when mctp can
have MCTP_NEIGH_DISCOVER neighbours.

Signed-off-by: Gagan Kumar <gagan1kumar.cs@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>