arm64: mm: Correct the update of max_pfn

Hotplugged memory can be smaller than the original memory. For example,
on my target:

root@genericarmv8:~# cat /sys/kernel/debug/memblock/memory
   0: 0x0000000064005000..0x0000000064023fff    0 NOMAP
   1: 0x0000000064400000..0x00000000647fffff    0 NOMAP
   2: 0x0000000068000000..0x000000006fffffff    0 DRV_MNG
   3: 0x0000000088800000..0x0000000094ffefff    0 NONE
   4: 0x0000000094fff000..0x0000000094ffffff    0 NOMAP
max_pfn will affect read_page_owner. Therefore, it should first compare and
then select the larger value for max_pfn.

Fixes: 8fac67c ("arm64: mm: update max_pfn after memory hotplug")
Cc: <stable@vger.kernel.org> # 6.1.x
Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20250321070019.1271859-1-quic_zhenhuah@quicinc.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: Zhenhua Huang

arch/arm64/mm/mmu.c

@@ -1358,7 +1358,8 @@ int arch_add_memory(int nid, u64 start, u64 size,
 		__remove_pgd_mapping(swapper_pg_dir,
 				     __phys_to_virt(start), size);
 	else {
-		max_pfn = PFN_UP(start + size);
+		/* Address of hotplugged memory can be smaller */
+		max_pfn = max(max_pfn, PFN_UP(start + size));
 		max_low_pfn = max_pfn;
 	}