Silently ignore permission errors

Those usually occur on special filesystems which we don't care about
anyway.

df2db

@@ -12,6 +12,7 @@ csr = db.cursor()
 with open("/proc/mounts") as mounts:
     for mount in mounts:
         (dev, mountpoint, fstype, rest) = mount.split(' ', 3)
+        try:
             s = os.statvfs(mountpoint)
             if s.f_blocks == 0:
                 continue # not a real file system
@@ -40,6 +41,9 @@ with open("/proc/mounts") as mounts:
                     (hostname, mountpoint,
                     s_total, s_usable, s_used,
                     f_total, f_usable, f_used))
+        except PermissionError:
+            # silently ignore some errors
+            pass
 
 db.commit()