ovl: fix NULL pointer defer when encoding non-decodable lower fid

A wrong return value from ovl_check_encode_origin() would cause
ovl_dentry_to_fid() to try to encode fid from NULL upper dentry.

Reported-by: syzbot+2208f82282740c1c8915@syzkaller.appspotmail.com
Fixes: 16aac5ad ("ovl: support encoding non-decodable file handles")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

fs/overlayfs/export.c

@@ -188,7 +188,7 @@ static int ovl_check_encode_origin(struct dentry *dentry)
 
 	/* Lower file handle for non-upper non-decodable */
 	if (!ovl_dentry_upper(dentry) && !decodable)
-		return 0;
+		return 1;
 
 	/* Upper file handle for pure upper */
 	if (!ovl_dentry_lower(dentry))