smb: server: let smbdirect_map_sges_from_iter() truncate the message boundary

smb: server: let smbdirect_map_sges_from_iter() truncate the message boundary

smbdirect_map_sges_from_iter() already handles the case that only
a limited number of sges are available. Its return value
is data_length and the remaining bytes in the iter are
remaining_data_length.

This is now much easier and will allow us to share
more code with the client soon.

Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
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>