[IPV6]: Fix the return value of ipv6_getsockopt

If CONFIG_NETFILTER if not selected when compile the kernel source code, 
ipv6_getsockopt will returen an EINVAL error if optname is not supported by
the kernel. But if CONFIG_NETFILTER is selected, ENOPROTOOPT error will 
be return.

This patch fix to always return ENOPROTOOPT error if optname argument of 
ipv6_getsockopt is not supported by the kernel.

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