libnvdimm, namespace: make min namespace size 4K

The arbitrary 4MB minimum namespace size turns out to be too large for some environments. Quoting Cheng-mean Liu: In the case of emulated NVDIMM devices in the VM environment, there are scenarios that NVDIMM device with much smaller sizes are desired, for example, we might use a single enumerated NVDIMM DAX device for representing each container layer, which in some cases could be just a few KBs size. PAGE_SIZE is the minimum where we can still support DAX of at least a single page.

DWDan Williams committed 8 years ago