block: implement runtime pm strategy

block: implement runtime pm strategy

When a request is added:
    If device is suspended or is suspending and the request is not a
    PM request, resume the device.

When the last request finishes:
    Call pm_runtime_mark_last_busy().

When pick a request:
    If device is resuming/suspending, then only PM request is allowed
    to go.

The idea and API is designed by Alan Stern and described here:
http://marc.info/?l=linux-scsi&m=133727953625963&w=2

Signed-off-by: Lin Ming <[email protected]>
Signed-off-by: Aaron Lu <[email protected]>
Acked-by: Alan Stern <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>