mnt: Only change user settable mount flags in remount
(bnc#891689, CVE-2014-5206, CVE-2014-5207).
mnt: Move the test for MNT_LOCK_READONLY from change_mount_flags
into do_remount (bnc#891689, CVE-2014-5206, CVE-2014-5207).
mnt: Correct permission checks in do_remount (bnc#891689,
CVE-2014-5206, CVE-2014-5207).