x86/e820: Fix handling of subpage regions when calculating nosave ranges in e820__reg...

x86/e820: Fix handling of subpage regions when calculating nosave ranges in e820__register_nosave_regions()

While debugging kexec/hibernation hangs and crashes, it turned out that
the current implementation of e820__register_nosave_regions() suffers from
multiple serious issues:

 - The end of last region is tracked by PFN, causing it to find holes
   that aren't there if two consecutive subpage regions are present

 - The nosave PFN ranges derived from holes are rounded out (instead of
   rounded in) which makes it inconsistent with how explicitly reserved
   regions are handled

Fix this by:

 - Treating reserved regions as if they were holes, to ensure consistent
   handling (rounding out nosave PFN ranges is more correct as the
   kernel does not use partial pages)

 - Tracking the end of the last RAM region by address instead of pages
   to detect holes more precisely

These bugs appear to have been introduced about ~18 years ago with the very
first version of e820_mark_nosave_regions(), and its flawed assumptions were
carried forward uninterrupted through various waves of rewrites and renames.

[ mingo: Added Git archeology details, for kicks and giggles. ]

Fixes: e8eff5ac294e ("[PATCH] Make swsusp avoid memory holes and reserved memory regions on x86_64")
Reported-by: Roberto Ricci <io@r-ricci.it>
Tested-by: Roberto Ricci <io@r-ricci.it>
Signed-off-by: Myrrh Periwinkle <myrrhperiwinkle@qtmlabs.xyz>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Len Brown <len.brown@intel.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250406-fix-e820-nosave-v3-1-f3787bc1ee1d@qtmlabs.xyz
Closes: https://lore.kernel.org/all/Z4WFjBVHpndct7br@desktop0a/