df: use open(2), not stat, to trigger automounting

df: use open(2), not stat, to trigger automounting

* src/df.c (main): When iterating over command-line arguments,
attempting to ensure each backing file system is mounted, use
open, not stat.  stat is no longer sufficient to trigger
automounting, in some cases.  Based on a suggestion from Ian Kent.
More details in http://bugzilla.redhat.com/497830