mm: compaction: clean up comment about async compaction in isolate_migratepages

Since commit 282722b0 ("mm, compaction: restrict async compaction to
pageblocks of same migratetype"), async direct compaction is restricted to
scan the pageblocks of same migratetype.  Correct the comment accordingly.

Link: https://lkml.kernel.org/r/20220418141253.24298-9-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Charan Teja Kalla <charante@codeaurora.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Pintu Kumar <pintu@codeaurora.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>