mm/hugetlb: use separate nodemask for bootmem allocations

Hugetlb boot allocation has used online nodes for allocation since commit
de55996d7188 ("mm/hugetlb: use online nodes for bootmem allocation").
This was needed to be able to do the allocations earlier in boot, before
N_MEMORY was set.

This might lead to a different distribution of gigantic hugepages across
NUMA nodes if there are memoryless nodes in the system.

What happens is that the memoryless nodes are tried, but then the memblock
allocation fails and falls back, which usually means that the node that
has the highest physical address available will be used (top-down
allocation). While this will end up getting the same number of hugetlb
pages, they might not be be distributed the same way. The fallback for
each memoryless node might not end up coming from the same node as the
successful round-robin allocation from N_MEMORY nodes.

While administrators that rely on having a specific number of hugepages
per node should use the hugepages=N:X syntax, it's better not to change
the old behavior for the plain hugepages=N case.

To do this, construct a nodemask for hugetlb bootmem purposes only,
containing nodes that have memory. Then use that for round-robin bootmem
allocations.

This saves some cycles, and the added advantage here is that hugetlb_cma
can use it too, avoiding the older issue of pointless attempts to create a
CMA area for memoryless nodes (which will also cause the per-node CMA area
size to be too small).

Link: https://lkml.kernel.org/r/20250402205613.3086864-1-fvdl@google.com
Fixes: de55996d7188 ("mm/hugetlb: use online nodes for bootmem allocation")
Signed-off-by: Frank van der Linden <fvdl@google.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Luiz Capitulino <luizcap@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>