net/ipv6: check for mistakenly passed in non-AF_INET6 sockaddrs

Same check as for IPv4, also do for IPv6.

(If you passed in a IPv4 sockaddr_in here, the sizeof check
 in the line before would have triggered already though.)

Signed-off-by: Marcus Meissner <meissner@suse.de>
Cc: Reinhard Max <max@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>