aio: fix build when migration is disabled

When "fs/aio: Add support to aio ring pages migration" was applied, it
broke the build when CONFIG_MIGRATION was disabled.  Wrap the migration
code with a test for CONFIG_MIGRATION to fix this and save a few bytes
when migration is disabled.
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>

fs/aio.c

@@ -181,6 +181,7 @@ static int aio_set_page_dirty(struct page *page)
 	return 0;
 }
 
+#if IS_ENABLED(CONFIG_MIGRATION)
 static int aio_migratepage(struct address_space *mapping, struct page *new,
 			struct page *old, enum migrate_mode mode)
 {
@@ -208,10 +209,13 @@ static int aio_migratepage(struct address_space *mapping, struct page *new,
 
 	return rc;
 }
+#endif
 
 static const struct address_space_operations aio_ctx_aops = {
 	.set_page_dirty = aio_set_page_dirty,
+#if IS_ENABLED(CONFIG_MIGRATION)
 	.migratepage	= aio_migratepage,
+#endif
 };
 
 static int aio_setup_ring(struct kioctx *ctx)