CVE-2015-5370: s4:rpc_server: correctly maintain dcesrv_connection->max_{recv,xmit}_frag

These values are controlled by the client but only in a range between
2048 and 5840 (including these values in 8 byte steps).
recv and xmit result always in same min value.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11344

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>
