smb3: minor cleanup of compound_send_recv

Trivial cleanup. Will make future multichannel code smaller
as well.

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>