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

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 <richard.weiyang@gmail.com>
CC: Mike Rapoport <rppt@kernel.org>
CC: David Hildenbrand <david@redhat.com>
CC: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20240808001415.6298-2-richard.weiyang@gmail.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>