docs: fix various Documentation/ paths in header files

Fix various Documentation/ paths in include/linux/.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Reviewed-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/debugfs.h

diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
index eb5c2ba..fc1b930 100644
--- a/include/linux/debugfs.h
+++ b/include/linux/debugfs.h
@@ -9,7 +9,7 @@
  *	2 as published by the Free Software Foundation.
  *
  *  debugfs is for people to use instead of /proc or /sys.
- *  See Documentation/DocBook/kernel-api for more details.
+ *  See Documentation/DocBook/filesystems for more details.
  */
 
 #ifndef _DEBUGFS_H_