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/basic/fs-util.c

@@ -23,6 +23,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <linux/magic.h>
 #include <time.h>
 #include <unistd.h>
 
@@ -721,6 +722,9 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags,
 
                 if (fstat(child, &st) < 0)
                         return -errno;
+                if ((flags & CHASE_NO_AUTOFS) &&
+                    fd_check_fstype(child, AUTOFS_SUPER_MAGIC) > 0)
+                        return -EREMOTE;
 
                 if (S_ISLNK(st.st_mode)) {
                         char *joined;