nvme - Fix BUF_KERNPROC() SMP race

nvme - Fix BUF_KERNPROC() SMP race

* BUF_KERNPROC() must be issued before we submit the request.  The subq
  lock is not sufficient to interlock request completion (which only needs
  the comq lock).

* Only occurs under extreme loads, probably due to an IPI or Xinvltlb
  causing enough of a pause that the completion can run.  NVMe is so fast,
  probably no other controller would hit this particular race condition.

* Also fix a bio queueing race which can leave a bio hanging.  If no
  requests are available (which can only happen under very heavy I/O
  loads), the signaling to the admin thread on the next I/O completion
  can race the queueing of the bio.  Fix the race by making sure the
  admin thread is signalled *after* queueing the bio.