NVMe: Add a module parameter to use a threaded interrupt

We're currently calling bio_endio from hard interrupt context.  This is
not a good idea for preemptible kernels as it will cause longer latencies.
Using a threaded interrupt will run the entire queue processing mechanism
(including bio_endio) in a thread, which can be preempted.  Unfortuantely,
it also adds about 7us of latency to the single-I/O case, so make it a
module parameter for the moment.

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