First round of code cleanup and reorganization. This is mostly code on
the slow path. Changes to the fast path will follow later.

• Use the standard ULD registration and activation mechanism offered by if_cxgbe. This eliminates all the code that managed the list of offload_device structures. This simplifies the CPL dispatch too by eliminating t4tom_cpl_handler_register_flag and associated code.

• Remove all unused or write-only fields from various structures (iscsi_socket, offload_device, cxgbei_ulp2_ddp_info, ulp_iscsi_info)

• Eliminate the two line wrappers around malloc/free. While here, switch to using M_CXGBE for all allocations.

• Simplify the page size settings in the chip (for iSCSI). This ULD "owns" these settings so it should simply write the values that it wants to the A_ULP_RX_ISCSI_TAGMASK and A_ULP_RX_ISCSI_PSZ registers. This eliminates the globals ddp_page_order[], ddp_page_shift[], page_idx and all related code.

• Maintain the per-adapter ULD state in one data structure instead of two. This consolidates struct offload_device and struct cxgbei_ulp2_ddp_info and into struct cxgbei_data, which is stored in adapter->iscsi_softc.

• Leave socket->so_emuldata alone, it exists for a different purpose (which is definitely not iSCSI). Store the per-socket offload state in a new field in struct icl_conn instead. (The new field exists only in this project branch and hasn't been reviewed for inclusion into head yet).

• Switch to the system version of mbufq.

• Tidy up the CPL/callback dispatch from if_cxgbe/t4_tom into cxgbei. The tid/toepcb is always available (t4_tom has looked it up) so it's a waste of time looking it up again.