[GFS2] Make headers compile on their own

As per Jan Engelhardt's comments, this should make all the headers
compile on their own by including and/or declaring structures
early.

Cc: Jan Engelhardt <jengelh@linux01.gwdg.de>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/incore.h

@@ -10,6 +10,8 @@
 #ifndef __INCORE_DOT_H__
 #define __INCORE_DOT_H__
 
+#include <linux/fs.h>
+
 #define DIO_FORCE	0x00000001
 #define DIO_CLEAN	0x00000002
 #define DIO_DIRTY	0x00000004