UPSTREAM: 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>
(cherry picked from commit f366d322aea782cf786aa821d5accdc1609f9e10)
Bug: 146400078
Bug: 148297388
Change-Id: I9eda3e848190b5bd26e5fc7f4d3cfdcb648fd815
Signed-off-by: Alistair Delva <adelva@google.com>