fs: nfs: inform the VM about pages being committed or unstable

fs: nfs: inform the VM about pages being committed or unstable

VM page reclaim uses dirty and writeback page states to determine if
flushers are cleaning pages too slowly and that page reclaim should stall
waiting on flushers to catch up.  Page state in NFS is a bit more complex
and a clean page can be unreclaimable due to being unstable which is
effectively "dirty" from the perspective of the VM from reclaim context.
Similarly, if the inode is currently being committed then it's similar to
being under writeback.

This patch adds a is_dirty_writeback() handled for NFS that checks if a
pages backing inode is being committed and should be accounted as
writeback and if a page has private state indicating that it is
effectively dirty.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Cc: Zlatko Calusic <zcalusic@bitsync.net>
Cc: dormando <dormando@rydia.net>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>