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: NCheng-mean Liu <soccerl@microsoft.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

include/uapi/linux/ndctl.h

@@ -209,7 +209,7 @@ enum nd_driver_flags {
 };
 
 enum {
-	ND_MIN_NAMESPACE_SIZE = 0x00400000,
+	ND_MIN_NAMESPACE_SIZE = PAGE_SIZE,
 };
 
 enum ars_masks {