Fix corner case when /etc/mtab doesn't exist and procfs=/proc (#1470)

In some Linux configurations the `/etc/mtab` does not exist but the
procfs_path is equal to `/proc`.
With the fix done for the issue #1307, the described configuration didn't work.

This Commit introduce an additional check that verifies if the
`/etc/mtab` file exists before using it, else it defaults to `<procfs_path>/self/mounts`

Signed-off-by: cedric lamoriniere <cedric.lamoriniere@datadoghq.com>

psutil/_pslinux.py

@@ -1169,13 +1169,13 @@ def disk_partitions(all=False):
                     fstypes.add("zfs")
 
     # See: https://github.com/giampaolo/psutil/issues/1307
-    if procfs_path == "/proc":
-        mtab_path = os.path.realpath("/etc/mtab")
+    if procfs_path == "/proc" and os.path.isfile('/etc/mtab'):
+        mounts_path = os.path.realpath("/etc/mtab")
     else:
-        mtab_path = os.path.realpath("%s/self/mounts" % procfs_path)
+        mounts_path = os.path.realpath("%s/self/mounts" % procfs_path)
 
     retlist = []
-    partitions = cext.disk_partitions(mtab_path)
+    partitions = cext.disk_partitions(mounts_path)
     for partition in partitions:
         device, mountpoint, fstype, opts = partition
         if device == 'none':