kernel - Fix list corruption in dsched

kernel - Fix list corruption in dsched

* dsched_thread_ctx_alloc() was not acquiring the global lock across
  calls to dsched_thread_io_alloc(), creating a race condition in the
  call to TAILQ_INSERT_TAIL(&tdio->diskctx->tdio_list, ...).

  This case can occur under heavy fork/exit loads, particularly when
  (soon) fork and exit get out from the MP lock, but even with the MP
  lock on the frontend it can occur against the dsched backend.

* dsched_new_policy_thread_tdio() was not acquiring the global lock
  across its initial call to dsched_thread_io_alloc().  This case could
  only occur with extreme rarity.