identify multipage ->writepages() calls

 NFS needs to be able to distinguish between single-page ->writepage() calls and
 multipage ->writepages() calls.

 For the single-page writepage calls NFS can kick off the I/O within the
 context of ->writepage().

 For multipage ->writepages calls, nfs_writepage() will leave the I/O pending
 and nfs_writepages() will kick off the I/O when it all has been queued up
 within NFS.

 Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
 Signed-off-by: Andrew Morton <akpm@osdl.org>
 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>