[IPv6] SNMP: Restore Udp6InErrors incrementation

As the checksum verification is postponed till user calls recv or poll,
the inrementation of Udp6InErrors counter should be also postponed.
Currently, it is postponed in non-blocking operation case. However it
should be postponed in all case like the IPv4 code.

Signed-off-by: Mitsuru Chinen <mitch@linux.vnet.ibm.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>