memblock, arm: fix crashes caused by holes in the memory map

The coordination between freeing of unused memory map, pfn_valid() and core
mm assumptions about validity of the memory map in various ranges was not
designed for complex layouts of the physical memory with a lot of holes all
over the place.

Kefen Wang reported crashes in move_freepages() on a system with the
following memory layout [1]:

  node   0: [mem 0x0000000080a00000-0x00000000855fffff]
  node   0: [mem 0x0000000086a00000-0x0000000087dfffff]
  node   0: [mem 0x000000008bd00000-0x000000008c4fffff]
  node   0: [mem 0x000000008e300000-0x000000008ecfffff]
  node   0: [mem 0x0000000090d00000-0x00000000bfffffff]
  node   0: [mem 0x00000000cc000000-0x00000000dc9fffff]
  node   0: [mem 0x00000000de700000-0x00000000de9fffff]
  node   0: [mem 0x00000000e0800000-0x00000000e0bfffff]
  node   0: [mem 0x00000000f4b00000-0x00000000f6ffffff]
  node   0: [mem 0x00000000fda00000-0x00000000ffffefff]

These crashes can be mitigated by enabling CONFIG_HOLES_IN_ZONE on ARM and
essentially turning pfn_valid_within() to pfn_valid() instead of having it
hardwired to 1 on that architecture, but this would require to keep
CONFIG_HOLES_IN_ZONE solely for this purpose.

A cleaner approach is to update ARM's implementation of pfn_valid() to take
into accounting rounding of the freed memory map to pageblock boundaries
and make sure it returns true for PFNs that have memory map entries even if
there is no physical memory backing those PFNs.

[1] https://lore.kernel.org/lkml/2a1592ad-bc9d-4664-fd19-f7448a37edc0@huawei.com
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