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 --cc fs/gfs2/export.c
Simple merge

diff --cc fs/gfs2/glock.c
Simple merge

diff --cc fs/gfs2/incore.h
Simple merge

diff --cc fs/gfs2/inode.c
Simple merge

diff --cc fs/gfs2/inode.h
Simple merge

diff --cc fs/gfs2/log.c
Simple merge

diff --cc fs/gfs2/log.h
Simple merge

diff --cc fs/gfs2/super.c
Simple merge