afs: Fix potential null pointer dereference in afs_put_server

commit 9158c6b upstream.

afs_put_server() accessed server->debug_id before the NULL check, which
could lead to a null pointer dereference. Move the debug_id assignment,
ensuring we never dereference a NULL server pointer.

Fixes: 2757a4d ("afs: Fix access after dec in put functions")
Cc: stable@vger.kernel.org
Signed-off-by: Zhen Ni <zhen.ni@easystack.cn>
Acked-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: nizhen-t

fs/afs/server.c

@@ -394,13 +394,14 @@ struct afs_server *afs_use_server(struct afs_server *server, enum afs_server_tra
 void afs_put_server(struct afs_net *net, struct afs_server *server,
 		    enum afs_server_trace reason)
 {
-	unsigned int a, debug_id = server->debug_id;
+	unsigned int a, debug_id;
 	bool zero;
 	int r;
 
 	if (!server)
 		return;
 
+	debug_id = server->debug_id;
 	a = atomic_read(&server->active);
 	zero = __refcount_dec_and_test(&server->ref, &r);
 	trace_afs_server(debug_id, r - 1, a, reason);