UAPI: ndctl: Remove use of PAGE_SIZE

The macro PAGE_SIZE isn't valid outside of the kernel, so it should not
appear in UAPI headers.

Furthermore, the actual machine page size could theoretically change from
an application's point of view if it's running in a container that gets
migrated to another machine (say 4K/ppc64 to 64K/ppc64).

Fixes: f2ba5a5b ("libnvdimm, namespace: make min namespace size 4K")
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>