mm: remove MIGRATE_ISOLATE check in hotpath

mm: remove MIGRATE_ISOLATE check in hotpath

Several functions test MIGRATE_ISOLATE and some of those are hotpath but
MIGRATE_ISOLATE is used only if we enable CONFIG_MEMORY_ISOLATION(ie, CMA,
memory-hotplug and memory-failure) which are not common config option.  So
let's not add unnecessary overhead and code when we don't enable
CONFIG_MEMORY_ISOLATION.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>