mm/mm_init.c: add new function calc_nr_all_pages()

mm/mm_init.c: add new function calc_nr_all_pages()

This is a preparation to calculate nr_kernel_pages and nr_all_pages, both
of which will be used later in alloc_large_system_hash().

nr_all_pages counts up all free but not reserved memory in memblock
allocator, including HIGHMEM memory.  While nr_kernel_pages counts up all
free but not reserved low memory in memblock allocator, excluding HIGHMEM
memory.

Link: https://lkml.kernel.org/r/20240325145646.1044760-4-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>