mm: compaction: use helper compound_nr in isolate_migratepages_block

Use helper compound_nr to make use of compound_nr when CONFIG_64BIT and
simplify the code a bit.

Link: https://lkml.kernel.org/r/20220418141253.24298-8-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>