NFS: Flesh out nfs_invalidate_page()

In the case of a call to truncate_inode_pages(), we should really try to
cancel any pending writes on the page.

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

include/linux/nfs_fs.h

@@ -61,6 +61,7 @@
 #define FLUSH_LOWPRI		8	/* low priority background flush */
 #define FLUSH_HIGHPRI		16	/* high priority memory reclaim flush */
 #define FLUSH_NOCOMMIT		32	/* Don't send the NFSv3/v4 COMMIT */
+#define FLUSH_INVALIDATE	64	/* Invalidate the page cache */
 
 #ifdef __KERNEL__