[IPv6]: use container_of() macro in fib6_clean_node()

In ip6_fib.c, fib6_clean_node() casts a fib6_walker_t pointer to
a fib6_cleaner_t pointer assuming a struct fib6_walker_t (field 'w')
is the first field in struct fib6_walker_t.

To prevent any future problems that may occur if one day a field
is inadvertently inserted before the 'w' field in struct fib6_cleaner_t,
(and to improve readability), this patch uses the container_of() macro.

Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>