landlock: Improve variable scope

This is now possible thanks to the disconnected directory fix.

Cc: Günther Noack <[e-mail]>
Cc: Song Liu <[e-mail]>
Cc: Tingmao Wang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mickaël Salaün <[e-mail]>

security/landlock/fs.c

···
 	 * restriction.
 	 */
 	while (true) {
-		struct dentry *parent_dentry;
 		const struct landlock_rule *rule;
 
 		/*
···
 			walker_path.dentry = walker_path.mnt->mnt_root;
 			dget(walker_path.dentry);
 		} else {
-			parent_dentry = dget_parent(walker_path.dentry);
+			struct dentry *const parent_dentry =
+				dget_parent(walker_path.dentry);
+
 			dput(walker_path.dentry);
 			walker_path.dentry = parent_dentry;
 		}