net/ipv6: switch ipv6_flowlabel_opt to sockptr_t

Pass a sockptr_t to prepare for set_fs-less handling of the kernel
pointer from bpf-cgroup.

Note that the get case is pretty weird in that it actually copies data
back to userspace from setsockopt.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>