mm/page_alloc: remove the zone_spans_pfn() check from get_pfnblock_bitmap_bitidx()

删除 get_pfnblock_bitmap_bitidx() 中未加 span_seqlock 的 zone_spans_pfn() 调试检查，避免内存热插拔调整 zone 边界时与并发 free_unref_folios() 竞争触发 VM_BUG_ON_PAGE；该检查只是 debug sanity。