smb: client: let smbd_post_send_iter() get remaining_length and return data_length

smb: client: let smbd_post_send_iter() get remaining_length and return data_length

This lets the logic be like smb_direct_post_send_data(), so
we can share common code in the next steps.

Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Long Li <longli@microsoft.com>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>