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.

[backport of net-next f95bf346226b9b79352e05508beececc807cc37a]

Change-Id: I86665b3f4e17c29afd496571d30cec7d14672009
Tested: https://android-review.googlesource.com/269874


Fixes: 5d77dca82839 ("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>