kernel/fork.c: get estimated free pages by memblock api

Instead of getting estimated free pages from memblock directly, we have
introduced an API, memblock_estimated_nr_free_pages(), which is more
friendly for users.

Just replace it with new API, no functional change.

Signed-off-by: Wei Yang <[email protected]>
CC: Mike Rapoport <[email protected]>
CC: David Hildenbrand <[email protected]>
CC: Oleg Nesterov <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mike Rapoport (Microsoft) <[email protected]>

Author: RichardWeiYang

kernel/fork.c

@@ -997,7 +997,7 @@ void __init __weak arch_task_cache_init(void) { }
 static void __init set_max_threads(unsigned int max_threads_suggested)
 {
 	u64 threads;
-	unsigned long nr_pages = PHYS_PFN(memblock_phys_mem_size() - memblock_reserved_size());
+	unsigned long nr_pages = memblock_estimated_nr_free_pages();
 
 	/*
 	 * The number of threads shall be limited such that the thread