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>

fs/gfs2/incore.h

@@ -20,7 +20,6 @@
 
 #define DIO_WAIT	0x00000010
 #define DIO_METADATA	0x00000020
-#define DIO_ALL		0x00000100
 
 struct gfs2_log_operations;
 struct gfs2_log_element;
@@ -377,8 +376,6 @@ struct gfs2_ail {
 	unsigned int ai_first;
 	struct list_head ai_ail1_list;
 	struct list_head ai_ail2_list;
-
-	u64 ai_sync_gen;
 };
 
 struct gfs2_journal_extent {
@@ -657,7 +654,6 @@ struct gfs2_sbd {
 	spinlock_t sd_ail_lock;
 	struct list_head sd_ail1_list;
 	struct list_head sd_ail2_list;
-	u64 sd_ail_sync_gen;
 
 	/* Replay stuff */