tmpfiles: silently ignore any path that passes through autofs (#6506)

If a path passes though an autofs filesystem, then accessing
the path might trigger and automount.  As systemd-tmpfiles is run before
the network is up, and as automounts are often used for networked
filesystems, this can cause a deadlock.

So chase_symlinks is enhance to accept a new flag which tells it
to check for autofs, and return -EREMOTE if autofs is found.

tmpfiles is changed to check just before acting on a path so that it
can avoid autofs even if a symlink was created earlier by tmpfiles
that would send this path through an autofs.

This fixes a deadlock that happens when /home is listed in /etc/fstab as
x-systemd.automount for an NFS directory.

src/tmpfiles/tmpfiles.c

@@ -1664,6 +1664,9 @@ static int process_item(Item *i) {
                 }
         }
 
+        if (chase_symlinks(i->path, NULL, CHASE_NO_AUTOFS, NULL) == -EREMOTE)
+                return t;
+
         r = arg_create ? create_item(i) : 0;
         q = arg_remove ? remove_item(i) : 0;
         p = arg_clean ? clean_item(i) : 0;