[IPV6] fix ipv6_getsockopt_sticky copy_to_user leak

User supplied len < 0 can cause leak of kernel memory.
Use unsigned compare instead.

Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>