CONFIG_DEBUG_FS=y and CONFIG_SYSFS=n results in the following compile
error:

<--  snip  -->

...
  LD      vmlinux
fs/built-in.o: In function `debugfs_init':
inode.c:(.init.text+0x31be): undefined reference to `kernel_subsys'
make: *** [vmlinux] Error 1

<--  snip  -->
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>