NVMe: Allow fatal signals to interrupt I/O

If the user sends a fatal signal, sleeping in the TASK_KILLABLE state
permits the task to be aborted.  The only wrinkle is making sure that
if/when the command completes later that it doesn't upset anything.
Handle this by setting the data pointer to 0, and checking the value
isn't NULL in the sync completion path.  Eventually, bios can be cancelled
through this path too.  Note that the cmdid isn't freed to prevent reuse.

We should also abort the command in the future, but this is a good start.

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