net: fix tiny output corruption of /proc/net/snmp6

Because "name" is static, it can be occasionally be filled with
somewhat garbage if two processes read /proc/net/snmp6.

Also, remove useless casts and "-1" -- snprintf() correctly terminates it's
output.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>