udp: Fix the SNMP counter of UDP_MIB_INERRORS

UDP packets received in udpv6_recvmsg() are not only IPv6 UDP packets, but
also have IPv4 UDP packets, so when do the counter of UDP_MIB_INERRORS in
udpv6_recvmsg(), we should check whether the packet is a IPv6 UDP packet
or a IPv4 UDP packet.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>