df: improve mount point selection with inaccurate mount list

df: improve mount point selection with inaccurate mount list

v8.23 has a test failure on Fedora rawhide build servers
in tests/df/skip-duplicate.sh.  This was due to no '/'
entry being output by df.  That was due to an inaccurate
/proc/mounts on the build environment as stat(/mnt/point)
identified all these /proc/mounts entries as having the
same device id:

  /                    rootfs
  /                    /dev/md1
  /dev                 devtmpfs
  /run                 tmpfs
  /boot                /dev/md0
  /proc/filesystems    /dev/md1

Since the device name on the right changes for a given id,
that causes the entries to be continually replaced, thus
resulting in no '/' entry.  I'm guessing this is due to
the mock environment bind mounting unneeded or sensitive
items to a dummy file on the host / (/dev/md1) though
have not looked into those details.

So rather than relying on an accurate /proc/mounts,
the attached patch takes a more conservative replacement
approach and only swaps a new device entry when the
mount point matches.  That should handle all practical
cases while also avoiding this situation.

* src/df.c (filter_mount_list): Only replace entries with
different device names when the mount point also matches.