ipv6/udp: report SndbufErrors and RcvbufErrors

commit a18135eb (Add UDP_MIB_{SND,RCV}BUFERRORS handling.)
forgot to make the necessary changes in net/ipv6/proc.c to report
additional counters in /proc/net/snmp6

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>