cxl: Clarify comment in spa_maps_hpa()

cxl: Clarify comment in spa_maps_hpa()

Update the comment in spa_maps_hpa() to clearly convey the construction
of extended linear cache.

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/linux-cxl/68eea19c7e67e_2f899100a8@dwillia2-mobl4.notmuch/
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Gregory Price <gourry@gourry.net>
Link: https://patch.msgid.link/20251106170108.1468304-3-dave.jiang@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>