mm/memory_hotplug: make shrink_zone_span() more robust

Let's make shrink_zone_span() more robust by checking in
find_smallest_section_pfn() / find_biggest_section_pfn() that the
start or end PFN of the subsection is within the zone.

While at it, clean up the function by factoring the core check out
into subsection_overlaps_zone().

There likely is no need to check the nid first. We require
SPARSEMEM_VMEMMAP_ENABLE, where pfn_to_page() is cheap, and
pfn_to_nid() on CONFIG_NUMA would call pfn_to_page() either way.
So let's just drop that for now.

Tested-by: Yuan Liu <yuan1.liu@intel.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Yuan Liu <yuan1.liu@intel.com>
Link: https://patch.msgid.link/20260920084946.3266279-2-yuan1.liu@intel.com
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

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