x86/mm/ident_map: Use gbpages only where full GB page should be mapped.

When ident_pud_init() uses only gbpages to create identity maps, large
ranges of addresses not actually requested can be included in the
resulting table; a 4K request will map a full GB.  On UV systems, this
ends up including regions that will cause hardware to halt the system
if accessed (these are marked "reserved" by BIOS).  Even processor
speculation into these regions is enough to trigger the system halt.

Only use gbpages when map creation requests include the full GB page
of space.  Fall back to using smaller 2M pages when only portions of a
GB page are included in the request.

No attempt is made to coalesce mapping requests. If a request requires
a map entry at the 2M (pmd) level, subsequent mapping requests within
the same 1G region will also be at the pmd level, even if adjacent or
overlapping such requests could have been combined to map a full
gbpage.  Existing usage starts with larger regions and then adds
smaller regions, so this should not have any great consequence.

[ dhansen: fix up comment formatting, simplifty changelog ]

Signed-off-by: Steve Wahl <steve.wahl@hpe.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20240126164841.170866-1-steve.wahl%40hpe.com