x86/boot/e820: Make sure e820_search_gap() finds all gaps

x86/boot/e820: Make sure e820_search_gap() finds all gaps

The current implementation of e820_search_gap() searches gaps
in a reverse search from MAX_GAP_END back to 0, contrary to
what its main comment claims:

    * Search for a gap in the E820 memory space from 0 to MAX_GAP_END (4GB).

But gaps can not only be beyond E820 RAM ranges, they can be below
them as well. For example this function will not find the proper
PCI gap for simplified memory map layouts that have a single RAM
range that crosses the 4GB boundary.

Rework the function to have a proper forward search of
E820 table entries.

This makes the code somewhat bigger:

   text       data        bss        dec        hex    filename
   7613      44072          0      51685       c9e5    e820.o.before
   7645      44072          0      51717       ca05    e820.o.after

but it now both implements what it claims to do, and is more
straightforward to read.

( This also allows 'idx' to be the regular u32 again, not an 'int'
  underflowing to -1. )

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: H . Peter Anvin <hpa@zytor.com>
Cc: Andy Shevchenko <andy@kernel.org>
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Juergen Gross <jgross@suse.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Paul Menzel <pmenzel@molgen.mpg.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://patch.msgid.link/20250515120549.2820541-29-mingo@kernel.org