doc: move seg6_flowlabel to seg6-sysctl.rst

Let's have all seg6 sysctl at the same place.

Fixes: a6dc6670 ("ipv6: sr: Add documentation for seg_flowlabel sysctl")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>