statefiles: use dirfd in dhcpv6_ia_write_statefile()

statefiles: use dirfd in dhcpv6_ia_write_statefile()

Similarly to the hostsfile, use a dirfd to keep track of the directory
where the statefile is stored, simplifying the code a bit and avoiding
some more allocations.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/302
Signed-off-by: Álvaro Fernández Rojas <[email protected]>