Use newly added extended LUN support in CAM.

Sponsored by:	The FreeBSD Foundation
Fix harmless warning after reconnecting a session and not doing anything
with it.

Sponsored by:	The FreeBSD Foundation
Move the ic_outstanding_count under #ifdef DIAGNOSTIC.

Sponsored by:	The FreeBSD Foundation
Move the atpit device model from userspace into vmm.ko for better
precision and lower latency.

Approved by:	grehan (co-mentor)