NVMe: Set TASK_INTERRUPTIBLE before processing queues

The kthread has two tasks; handling timeouts (for which it runs once per
second), and submitting queued BIOs.  If a BIO happens to be queued after
the thread has processed the queue but before it calls schedule_timeout(),
the thread will sleep for a second before submitting it, which can cause
performance problems in some rare cases (that will become more common in
a subsequent patch).

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Tested-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>