firewire: Add ref-counting for sbp2 orbs (fix command abortion)

This handles the case where we get the status write before getting the
complete_transaction callback ("status write for unknown orb").  In
this case, we just assume that the initial orb pointer transaction
succeeded and finish the orb.  To prevent the transaction callback
from touching freed memory, we ref-count the orb structures.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>