fs: rcu-walk aware d_revalidate method

Require filesystems be aware of .d_revalidate being called in rcu-walk
mode (nd->flags & LOOKUP_RCU). For now do a simple push down, returning
-ECHILD from all implementations.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>
diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index c9a8a42..07f4b5e 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -156,8 +156,12 @@
  */
 static int fuse_dentry_revalidate(struct dentry *entry, struct nameidata *nd)
 {
-	struct inode *inode = entry->d_inode;
+	struct inode *inode;
 
+	if (nd->flags & LOOKUP_RCU)
+		return -ECHILD;
+
+	inode = entry->d_inode;
 	if (inode && is_bad_inode(inode))
 		return 0;
 	else if (fuse_dentry_time(entry) < get_jiffies_64()) {