NVMe: Use an IDA to allocate minor numbers

The current approach of using the namespace ID as the minor number
doesn't work when there are multiple adapters in the machine.  Rather
than statically partitioning the number of namespaces between adapters,
dynamically allocate minor numbers to namespaces as they are detected.

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