[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/eaops.h

@@ -11,6 +11,7 @@
 #define __EAOPS_DOT_H__
 
 struct gfs2_ea_request;
+struct gfs2_inode;
 
 struct gfs2_eattr_operations {
 	int (*eo_get) (struct gfs2_inode *ip, struct gfs2_ea_request *er);