[IPV4]: Clean up fib_hash.c list handling.

1) Use linux/list.h facilities instead of by-hand
   list implementation.

2) Use fib_alias structure, hung off of fib_node,
   to represent routes which all have the same
   destination prefix, yet have different TOS and
   priority values.
Signed-off-by: David S. Miller <davem@davemloft.net>