mm/memory_hotplug: remove a wrapper for alloc_migration_target()

mm/memory_hotplug: remove a wrapper for alloc_migration_target()

To calculate the correct node to migrate the page for hotplug, we need to
check node id of the page.  Wrapper for alloc_migration_target() exists
for this purpose.

However, Vlastimil informs that all migration source pages come from a
single node.  In this case, we don't need to check the node id for each
page and we don't need to re-set the target nodemask for each page by
using the wrapper.  Set up the migration_target_control once and use it
for all pages.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Roman Gushchin <guro@fb.com>
Link: http://lkml.kernel.org/r/1594622517-20681-10-git-send-email-iamjoonsoo.kim@lge.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>