cam: create xpt_sim_poll and refactor
xpt_sim_poll takes the sim to poll as an argument. It will do the
proper locking protocol, call the SIM polling routine, and then call
camisr_runqueue to process completions on any CCBs the SIM's poll
routine completed. It will be used during late shutdown when a SIM is
waiting for CCBs it sent during shutdown to finish and the scheduler
isn't running because we've panic'd.
This sequence was used twice in cam_xpt, so refactor those to use this
mps: Call xpt_sim_poll in shutdown_final handler.
When we're shutting down, we send a number of start/stop commands to
the known targets. We have to wait for them to complete. During a
panic, the interrupts are off, and using pause to wait for them to
fire and complete won't work: we have to poll after pause returns so
the completion routines of the CCBs run so we decrement work
mpr: Port the mps panic-safe shutdown_final handling
r330951 by smh fixed the mps driver to avoid deadlocks when panicing.
The same code is needed for mpr, so port it here, along with the fix
which allows the CCBs scheduled to complete avoiding at least a scary
message and likely other unintended consequences.