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.

Cc: Matthew Wilcox <willy@infradead.org>
Reported-by: Cheng-mean Liu <soccerl@microsoft.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>