nvdimm/region: Fix default alignment for small regions

nvdimm/region: Fix default alignment for small regions

In preparation for removing BLK aperture support the NVDIMM unit tests
discovered that the default alignment can be set higher than the
capacity of the region. Fall back to PAGE_SIZE in that case.

Given this has not been seen in the wild, elide notifying -stable.

Fixes: 2522afb86a8c ("libnvdimm/region: Introduce an 'align' attribute")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/164688416128.2879318.17890707310125575258.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>