mm: kill find_min_pfn_with_active_regions()

find_min_pfn_with_active_regions() is only called from free_area_init(). 
Open-code the PHYS_PFN(memblock_start_of_DRAM()) into free_area_init(),
and kill find_min_pfn_with_active_regions().

Link: https://lkml.kernel.org/r/20220815111017.39341-1-wangkefeng.wang@huawei.com


Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>