ipv6: Fix fib6_dump_table walker leak

[ Upstream commit: 7891cc818967e186be68caac32d84bfd0a3f0bd2 ]

When a fib6 table dump is prematurely ended, we won't unlink
its walker from the list.  This causes all sorts of grief for
other users of the list later.

Reported-by: Chris Caputo <ccaputo@alt.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>