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>