NVMe: Simplify completion handling

Instead of encoding the handler type in the bottom two bits of the
per-completion context pointer, store the handler function as well
as the context pointer.  This gives us more flexibility and the code
is clearer.  It comes at the cost of an extra 8k of memory per queue,
but this feels like a reasonable price to pay.

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