dax/hmem: Reduce visibility of dax_cxl coordination symbols

dax/hmem: Reduce visibility of dax_cxl coordination symbols

No other module or use case should be using dax_hmem_initial_probe or
dax_hmem_flush_work(). Limit their use to dax_hmem, and dax_cxl
respectively.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260327052821.440749-6-dan.j.williams@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>