memremap: provide stubs for vmem_altmap_offset and vmem_altmap_free

Currently all calls to those functions are eliminated by the compiler when
CONFIG_ZONE_DEVICE is not set, but this soon won't be the case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>