mm/migration: make isolate_movable_page always defined

Define isolate_movable_page as a static inline function when
CONFIG_MIGRATION is not enable.  It should return -EBUSY here which
means failed to isolate movable pages.

This patch do not have any functional change but prepare for later
patch.

Link: http://lkml.kernel.org/r/1485867981-16037-3-git-send-email-ysxie@foxmail.comSigned-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Suggested-by: Michal Hocko <mhocko@kernel.org>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Reza Arbab <arbab@linux.vnet.ibm.com>
Cc: Taku Izumi <izumi.taku@jp.fujitsu.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/migrate.h

@@ -56,6 +56,8 @@ static inline int migrate_pages(struct list_head *l, new_page_t new,
 		free_page_t free, unsigned long private, enum migrate_mode mode,
 		int reason)
 	{ return -ENOSYS; }
+static inline int isolate_movable_page(struct page *page, isolate_mode_t mode)
+	{ return -EBUSY; }
 
 static inline int migrate_prep(void) { return -ENOSYS; }
 static inline int migrate_prep_local(void) { return -ENOSYS; }