mm: memory_hotplug: override memmap_on_memory when hugetlb_free_vmemmap=on

Optimizing HugeTLB vmemmap pages is not compatible with allocating memmap
on hot added memory.  If "hugetlb_free_vmemmap=on" and
memory_hotplug.memmap_on_memory" are both passed on the kernel command
line, optimizing hugetlb pages takes precedence.  However, the global
variable memmap_on_memory will still be set to 1, even though we will not
try to allocate memmap on hot added memory.

Also introduce mhp_memmap_on_memory() helper to move the definition of
"memmap_on_memory" to the scope of CONFIG_MHP_MEMMAP_ON_MEMORY.  In the
next patch, mhp_memmap_on_memory() will also be exported to be used in
hugetlb_vmemmap.c.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Muchun Song <[email protected]>
Acked-by: Mike Kravetz <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Iurii Zaikin <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Xiongchun Duan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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