nfs: if we have no valid attrs, then don't declare the attribute cache valid

commit c812012f upstream.

If we pass in an empty nfs_fattr struct to nfs_update_inode, it will
(correctly) not update any of the attributes, but it then clears the
NFS_INO_INVALID_ATTR flag, which indicates that the attributes are
up to date. Don't clear the flag if the fattr struct has no valid
attrs to apply.
Reviewed-by: Steve French <steve.french@primarydata.com>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

fs/nfs/inode.c

@@ -1704,7 +1704,11 @@ static int nfs_update_inode(struct inode *inode, struct nfs_fattr *fattr)
 			nfsi->attrtimeo_timestamp = now;
 		}
 	}
-	invalid &= ~NFS_INO_INVALID_ATTR;
+
+	/* Don't declare attrcache up to date if there were no attrs! */
+	if (fattr->valid != 0)
+		invalid &= ~NFS_INO_INVALID_ATTR;
+
 	/* Don't invalidate the data if we were to blame */
 	if (!(S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode)
 				|| S_ISLNK(inode->i_mode)))