[UDP]: Move udp_stats_in6 into net/ipv4/udp.c

Now that external users may increment the counters directly, we need
to ensure that udp_stats_in6 is always available.  Otherwise we'd
either have to requrie the external users to be built as modules or
ipv6 to be built-in.

This isn't too bad because udp_stats_in6 is just a pair of pointers
plus an EXPORT, e.g., just 40 (16 + 24) bytes on x86-64.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>