NFSDv4: use export cache flushtime for changeid on V4ROOT objects.

If you change the set of filesystems that are exported, then
the contents of various directories in the NFSv4 pseudo-root
is likely to change.  However the change-id of those
directories is currently tied to the underlying directory,
so the client may not see the changes in a timely fashion.

This patch changes the change-id number to be derived from the
"flush_time" of the export cache.  Whenever any changes are
made to the set of exported filesystems, this flush_time is
updated.  The result is that clients see changes to the set
of exported filesystems much more quickly, often immediately.
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4xdr.c

@@ -1966,9 +1966,13 @@ nfsd4_decode_compound(struct nfsd4_compoundargs *argp)
 	DECODE_TAIL;
 }
 
-static __be32 *encode_change(__be32 *p, struct kstat *stat, struct inode *inode)
+static __be32 *encode_change(__be32 *p, struct kstat *stat, struct inode *inode,
+			     struct svc_export *exp)
 {
-	if (IS_I_VERSION(inode)) {
+	if (exp->ex_flags & NFSEXP_V4ROOT) {
+		*p++ = cpu_to_be32(convert_to_wallclock(exp->cd->flush_time));
+		*p++ = 0;
+	} else if (IS_I_VERSION(inode)) {
 		p = xdr_encode_hyper(p, inode->i_version);
 	} else {
 		*p++ = cpu_to_be32(stat->ctime.tv_sec);
@@ -2493,7 +2497,7 @@ nfsd4_encode_fattr(struct xdr_stream *xdr, struct svc_fh *fhp,
 		p = xdr_reserve_space(xdr, 8);
 		if (!p)
 			goto out_resource;
-		p = encode_change(p, &stat, d_inode(dentry));
+		p = encode_change(p, &stat, d_inode(dentry), exp);
 	}
 	if (bmval0 & FATTR4_WORD0_SIZE) {
 		p = xdr_reserve_space(xdr, 8);