[POSIX] Pass path to df(1) reference rather than device

Otherwise this test fails for environments with active bind-mounts.  For
example, consider the following:

$ mount test.img testdir
$ mount --rbind /usr/src testdir/usr/src
$ chroot testdir /bin/bash
$ python3
Python 3.11.6 (main, Dec  5 2023, 11:03:00) [GCC 13.2.1 20230826] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import psutil, pprint
>>> pprint.pprint(psutil.disk_partitions(all=False))
[sdiskpart(device='/dev/loop0', mountpoint='/', fstype='ext4', opts='rw,relatime', maxfile=255, maxpath=4096),
 sdiskpart(device='/dev/loop0', mountpoint='/usr/src', fstype='ext4', opts='ro,noatime', maxfile=255, maxpath=4096)]
>>> pprint.pprint([psutil.disk_usage(x.mountpoint) for x in psutil.disk_partitions(all=False)])
[sdiskusage(total=20530814976, used=7411703808, free=12053757952, percent=38.1),
 sdiskusage(total=134679105536, used=42708791296, free=85081747456, percent=33.4)]
>>>
$ df / && df /usr/src
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/loop0      20049624 7237992  11771248  39% /
Filesystem     1K-blocks     Used Available Use% Mounted on
/dev/root      131522564 41707804  83087644  34% /usr/src

Note that psutil has the correct size data, but the wrong block device.
If we pass the path to df(1) instead of the incorrect block device, then
df(1) will return results from the correct filesystem.

psutil/tests/test_posix.py

@@ -421,9 +421,9 @@ def test_os_waitpid_bad_ret_status(self):
     @unittest.skipIf(AIX, "unreliable on AIX")
     @retry_on_failure()
     def test_disk_usage(self):
-        def df(device):
+        def df(path):
             try:
-                out = sh("df -k %s" % device).strip()
+                out = sh("df -k %s" % path).strip()
             except RuntimeError as err:
                 if "device busy" in str(err).lower():
                     raise self.skipTest("df returned EBUSY")
@@ -440,7 +440,7 @@ def df(device):
         for part in psutil.disk_partitions(all=False):
             usage = psutil.disk_usage(part.mountpoint)
             try:
-                total, used, free, percent = df(part.device)
+                total, used, free, percent = df(part.mountpoint)
             except RuntimeError as err:
                 # see:
                 # https://travis-ci.org/giampaolo/psutil/jobs/138338464