smb: client: add NULL check in automount_fullpath

commit f1e7a277a1736e12cc4bd6d93b8a5c439b8ca20c upstream.

page is checked for null in __build_path_from_dentry_optional_prefix
when tcon->origin_fullpath is not set. However, the check is missing when
it is set.
Add a check to prevent a potential NULL pointer dereference.

Signed-off-by: Ruben Devos <devosruben6@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/smb/client/namespace.c

@@ -146,6 +146,9 @@ static char *automount_fullpath(struct dentry *dentry, void *page)
 	}
 	spin_unlock(&tcon->tc_lock);
 
+	if (unlikely(!page))
+		return ERR_PTR(-ENOMEM);
+
 	s = dentry_path_raw(dentry, page, PATH_MAX);
 	if (IS_ERR(s))
 		return s;