x86/vmemmap: optimize for consecutive sections in partial populated PMDs

We can optimize in the case we are adding consecutive sections, so no
memset(PAGE_UNUSED) is needed.

In that case, let us keep track where the unused range of the previous
memory range begins, so we can compare it with start of the range to be
added.  If they are equal, we know sections are added consecutively.

For that purpose, let us introduce 'unused_pmd_start', which always holds
the beginning of the unused memory range.

In the case a section does not contiguously follow the previous one, we
know we can memset [unused_pmd_start, PMD_BOUNDARY) with PAGE_UNUSE.

This patch is based on a similar patch by David Hildenbrand:

https://lore.kernel.org/linux-mm/20200722094558.9828-10-david@redhat.com/

Link: https://lkml.kernel.org/r/20210309214050.4674-5-osalvador@suse.de
Signed-off-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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