[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/bmap.h

@@ -10,6 +10,10 @@
 #ifndef __BMAP_DOT_H__
 #define __BMAP_DOT_H__
 
+struct inode;
+struct gfs2_inode;
+struct page;
+
 int gfs2_unstuff_dinode(struct gfs2_inode *ip, struct page *page);
 int gfs2_block_map(struct inode *inode, u64 lblock, int *new, u64 *dblock, int *boundary);
 int gfs2_extent_map(struct inode *inode, u64 lblock, int *new, u64 *dblock, unsigned *extlen);