ksmbd: Use struct_size() to improve smb_direct_rdma_xmit()

Use struct_size() to calculate the number of bytes to allocate for a
new message.
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/server/transport_rdma.c

@@ -1405,8 +1405,8 @@ static int smb_direct_rdma_xmit(struct smb_direct_transport *t,
 	/* build rdma_rw_ctx for each descriptor */
 	desc_buf = buf;
 	for (i = 0; i < desc_num; i++) {
-		msg = kzalloc(offsetof(struct smb_direct_rdma_rw_msg, sg_list) +
-			      sizeof(struct scatterlist) * SG_CHUNK_SIZE, GFP_KERNEL);
+		msg = kzalloc(struct_size(msg, sg_list, SG_CHUNK_SIZE),
+			      GFP_KERNEL);
 		if (!msg) {
 			ret = -ENOMEM;
 			goto out;