NVMe: Dynamically allocate partition numbers

Some users need more than 64 partitions per device.  Rather than simply
increasing the number of partitions, switch to the dynamic partition
allocation scheme.

This means that minor numbers are not stable across boots, but since major
numbers aren't either, I cannot see this being a significant problem.

Tested-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>