mm/huge_memory: minor cleanup for split_huge_pages_all

[ Upstream commit a17206da ]

There is nothing to do if a zone doesn't have any pages managed by the
buddy allocator. So we should check managed_zone instead. Also if a thp
is found, there's no need to traverse the subpages again.

Link: https://lkml.kernel.org/r/20220704132201.14611-13-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Stable-dep-of: 2b7aa91b ("mm/huge_memory: use pfn_to_online_page() in split_huge_pages_all()")
Signed-off-by: Sasha Levin <sashal@kernel.org>