proc: snmp6: Use correct type in memset

Reading /proc/net/snmp6 yields bogus values on 32 bit kernels.
Use "u64" instead of "unsigned long" in sizeof().

Fixes: 4a4857b1 ("proc: Reduce cache miss in snmp6_seq_show")
Signed-off-by: Christian Perle <christian.perle@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>