Add "type" to nvme_request, signifying if its payload is a VADDR, UIO, or
NULL. This simplifies decisions around if/how requests are routed through
busdma.  It also paves the way for supporting unmapped bios.

Sponsored by:	Intel
Keep fwd_tag around for subsequent pcb lookups

For TIMEWAIT handling tcp_input may have to jump back for an additional
pass through pcblookup.  Prior to this change the fwd_tag had been
discarded after the first lookup, so a new connection attempt delivered
locally via 'ipfw fwd' would fail to find a match.

As of r248886 the tag will be detached and freed when passed to the
socket buffer.