ipv6: Remove in6addr_any alternatives.

Some code defines the IPv6 wildcard address as a local variable and
use it with memcmp() or ipv6_addr_equal().

Let's use in6addr_any and ipv6_addr_any() instead.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>