ksmbd: smbd: change the return value of get_sg_list

Make get_sg_list return EINVAL if there aren't
mapped scatterlists.

Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>