Revert "ipv6: per-netns exclusive flowlabel checks"

This reverts commit 4f523f15 which is
commit 0b0dff5b upstream.

It breaks the kernel abi and is not needed for Android systems, so
revert it.

Bug: 161946584
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: If9df46769d0f25fcdfe7f4b8df071ade3907fada