generic/459: Fix check for ro-remount in extN

generic/459: Fix check for ro-remount in extN

Currently ,freeze failure caused by the lack of space can not
guarantee to remount extN filesystem in read-only mode, and test
failed due to "ro" mount option not found. We can add a touch to
trigger the action which aborts journal and ro-remounts the fs.

[eguan: update commit log and comments a bit]

Signed-off-by: yang xu <xuyang.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>