NVMe: Add the nvme thread to the wait queue before waking it up

If the I/O was not completed by a single NVMe command, we add the
bio to the congestion list and wake up the kthread to resubmit it.
But the kthread calls remove_wait_queue() unconditionally, which
will oops if it's not on the wait queue.  So add the kthread to
the wait queue before waking it up.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>