memblock: make HugeTLB bootmem allocation work with KHO

Gigantic huge page allocation is somewhat broken currently when KHO is
used.

Firstly, they break KHO scratch size accounting. RSRV_KERN is used to
track how much memory is reserved for use by the kernel. Since
hugetlb::alloc_bootmem() calls the memblock_alloc*() APIs, the hugepages
allocated also get marked as RSRV_KERN.

Allocations marked RSRV_KERN are used by KHO to calculate how much
scratch space it should reserve to make sure the next kernel has enough
memory to boot when it is in scratch-only phase. Counting hugepages in
that blows up scratch size, and can lead to the scratch allocation
failing, making KHO unusable. This will show up when huge pages make up
more than 50% of the system, which is a fairly common use case.

Secondly, while not supported right now, huge pages are user memory and
can be preserved via KHO. The scratch spaces should not have any
preserved memory. Allocating hugepages from scratch (on a KHO boot) can
lead to them being un-preservable.

Introduce memblock_alloc_hugetlb(). This lets memblock tailor to the
needs of hugetb without exposing those details to the general allocation
routines.

First, it does not use mirrored memory for hugetlb. Mirrored memory is a
limited resource that is best saved for kernel data structures, not user
memory.

Second, if the free memory area found by memblock_find_in_range_node()
is a part of a KHO scratch area, the free area is not used. Allocation
is retried starting after the free area to ensure no hugepages come from
KHO scratch.

Third, it simplifies the argument list by baking in some hugetlb
assumptions like alignment and exact_nid. This also simplifies
allocation logic in alloc_bootmem().

Also introduce MEMBLOCK_RSRV_HUGETLB to mark reservations made for
HugeTLB. This will be used by KHO in future patches to correctly
calculate scratch sizes.

Refactor some of the preparation logic like kmemleak tracking and
accepting memory into a separate helper memblock_prep_allocation(), and
use it from both memblock_alloc_hugetlb() and the usual
memblock_alloc_range_nid().

Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Link: https://patch.msgid.link/20260801084833.1897543-21-pratyush@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

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