resolv: Fix ABA race in /etc/resolv.conf change detection [BZ #25420]

__resolv_conf_get_current should only record the initial file
change data if after verifying that file just read matches the
original measurement.  Fixes commit aef16cc8
("resolv: Automatically reload a changed /etc/resolv.conf file
[BZ #984]").

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>