NVMe: fix race condition in nvme_submit_sync_cmd()

If we have a race between the schedule timing out and the command
completing, we could have the task issuing the command exit
nvme_submit_sync_cmd() while the irq is running sync_completion().
If that happens, we could be corrupting memory, since the stack
that held 'cmdinfo' is no longer valid.

Fix this by always calling nvme_abort_cmd_info(). Once that call
completes, we know that we have either run sync_completion() if
the completion came in, or that we will never run it since we now
have special_completion() as the command callback handler.

Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>