[NET]: Introduce union in struct dst_entry to hold 'next' pointer

This patch introduces an anonymous union to nicely express the fact that all
objects inherited from struct dst_entry should access to the generic 'next'
pointer but with appropriate type verification.

This patch is a prereq before following patches.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>