Remove usage of the request substate machine for stp requests, and kill
the request substate infrastructure.

Similar to the previous conversions this adds the substates to the
primary state machine and arranges for the 'started' state to transition
to the proper stp substate.
Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>