GFS2: Make writeback more responsive to system conditions

This patch adds writeback_control to writing back the AIL
list. This means that we can then take advantage of the
information we get in ->write_inode() in order to set off
some pre-emptive writeback.

In addition, the AIL code is cleaned up a bit to make it
a bit simpler to understand.

There is still more which can usefully be done in this area,
but this is a good start at least.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
diff --git a/fs/gfs2/inode.h b/fs/gfs2/inode.h
index 842346e..8d1344a 100644
--- a/fs/gfs2/inode.h
+++ b/fs/gfs2/inode.h
@@ -102,7 +102,7 @@
 extern struct inode *gfs2_lookup_by_inum(struct gfs2_sbd *sdp, u64 no_addr,
 					 u64 *no_formal_ino,
 					 unsigned int blktype);
-extern struct inode *gfs2_ilookup(struct super_block *sb, u64 no_addr);
+extern struct inode *gfs2_ilookup(struct super_block *sb, u64 no_addr, int nonblock);
 
 extern int gfs2_inode_refresh(struct gfs2_inode *ip);