[BLOCK] iosched: fix setting of default io scheduler

With the recent reorg of the io scheduler selection, it unfortunately
became possible to select an io scheduler to be the default even if it
wasn't builtin. Fix this by requiring the default scheduler to be
builtin.
Signed-off-by: Jens Axboe <axboe@suse.de>

block/Kconfig.iosched

@@ -46,13 +46,13 @@ choice
 	  block devices.
 
 	config DEFAULT_AS
-		bool "Anticipatory" if IOSCHED_AS
+		bool "Anticipatory" if IOSCHED_AS=y
 
 	config DEFAULT_DEADLINE
-		bool "Deadline" if IOSCHED_DEADLINE
+		bool "Deadline" if IOSCHED_DEADLINE=y
 
 	config DEFAULT_CFQ
-		bool "CFQ" if IOSCHED_CFQ
+		bool "CFQ" if IOSCHED_CFQ=y
 
 	config DEFAULT_NOOP
 		bool "No-op"