NFS: Make nfs_updatepage() mark the page as dirty.

This will ensure that we can call set_page_writeback() from within
nfs_writepage(), which is always called with the page lock set.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/nfs_page.h

@@ -31,6 +31,7 @@
 #define PG_NEED_COMMIT		1
 #define PG_NEED_RESCHED		2
 #define PG_NEED_FLUSH		3
+#define PG_FLUSHING		4
 
 struct nfs_inode;
 struct nfs_page {