fs: fix is_mnt_ns_file()

Commit 1fa08ae ("nsfs: convert to path_from_stashed() helper") reused
nsfs dentry's d_fsdata, which no longer contains a pointer to
proc_ns_operations.

Fix the remaining use in is_mnt_ns_file().

Fixes: 1fa08ae ("nsfs: convert to path_from_stashed() helper")
Cc: [email protected] # v6.9
Signed-off-by: Miklos Szeredi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Acked-by: Al Viro <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: Miklos Szeredi

fs/namespace.c

@@ -2055,9 +2055,15 @@ SYSCALL_DEFINE1(oldumount, char __user *, name)
 
 static bool is_mnt_ns_file(struct dentry *dentry)
 {
+	struct ns_common *ns;
+
 	/* Is this a proxy for a mount namespace? */
-	return dentry->d_op == &ns_dentry_operations &&
-	       dentry->d_fsdata == &mntns_operations;
+	if (dentry->d_op != &ns_dentry_operations)
+		return false;
+
+	ns = d_inode(dentry)->i_private;
+
+	return ns->ops == &mntns_operations;
 }
 
 struct ns_common *from_mnt_ns(struct mnt_namespace *mnt)