[IPV6] ip6_fib.c: make code static

Make the following needlessly global code static:
- fib6_walker_lock
- struct fib6_walker_list
- fib6_walk_continue()
- fib6_walk()

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>