mm: cleanup: remove #ifdef CONFIG_MIGRATION

#ifdef in *.c file decrease source readability a bit.  removing is better.

This patch doesn't have any functional change.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/rmap.c b/mm/rmap.c
index 1099394..53c56da 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -50,6 +50,7 @@
 #include <linux/kallsyms.h>
 #include <linux/memcontrol.h>
 #include <linux/mmu_notifier.h>
+#include <linux/migrate.h>
 
 #include <asm/tlbflush.h>
 
@@ -818,8 +819,7 @@
 				spin_unlock(&mmlist_lock);
 			}
 			dec_mm_counter(mm, anon_rss);
-#ifdef CONFIG_MIGRATION
-		} else {
+		} else if (PAGE_MIGRATION) {
 			/*
 			 * Store the pfn of the page in a special migration
 			 * pte. do_swap_page() will wait until the migration
@@ -827,19 +827,15 @@
 			 */
 			BUG_ON(!migration);
 			entry = make_migration_entry(page, pte_write(pteval));
-#endif
 		}
 		set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
 		BUG_ON(pte_file(*pte));
-	} else
-#ifdef CONFIG_MIGRATION
-	if (migration) {
+	} else if (PAGE_MIGRATION && migration) {
 		/* Establish migration entry for a file page */
 		swp_entry_t entry;
 		entry = make_migration_entry(page, pte_write(pteval));
 		set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
 	} else
-#endif
 		dec_mm_counter(mm, file_rss);