NFS: Ensure inode is always marked I_DIRTY_DATASYNC, if it has unstable pages

Since nfs_scan_list() doesn't wait for locked pages, we have a race in
which it is possible to end up with an inode that needs to send a COMMIT,
but which does not have the I_DIRTY_DATASYNC flag set.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/write.c

@@ -582,6 +582,8 @@ nfs_scan_commit(struct inode *inode, struct list_head *dst, pgoff_t idx_start, u
 	ret = nfs_scan_list(nfsi, dst, idx_start, npages, NFS_PAGE_TAG_COMMIT);
 	if (ret > 0)
 		nfsi->ncommit -= ret;
+	if (nfs_need_commit(NFS_I(inode)))
+		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
 	return ret;
 }
 #else