net: diag: make udp_diag_destroy work for mapped addresses.

udp_diag_destroy does look up the IPv4 UDP hashtable for mapped
addresses, but it gets the IPv4 address to look up from the
beginning of the IPv6 address instead of the end.

Tested: https://android-review.googlesource.com/269874
Fixes: 5d77dca8

 ("net: diag: support SOCK_DESTROY for UDP sockets")
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>