statefiles: simplify statefiles_write_state6()

statefiles: simplify statefiles_write_state6()

By collecting the addresses first and writing the hosts entries, we can
write the state line straight to the file, without having to use a
buffer as an intermediary. This also makes the ordering in the function
more logical, as it reflects the order things get written to file.

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