s390/mem_detect: Use IS_ENABLED(CONFIG_BLK_DEV_INITRD)

IS_ENABLED should generally use CONFIG_ prefaced symbols and
it doesn't appear as if there is a BLK_DEV_INITRD define.

Cc: <stable@vger.kernel.org> # 4.20
Signed-off-by: NJoe Perches <joe@perches.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/boot/mem_detect.c

@@ -25,7 +25,7 @@ static void *mem_detect_alloc_extended(void)
 {
 	unsigned long offset = ALIGN(mem_safe_offset(), sizeof(u64));
 
-	if (IS_ENABLED(BLK_DEV_INITRD) && INITRD_START && INITRD_SIZE &&
+	if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && INITRD_START && INITRD_SIZE &&
 	    INITRD_START < offset + ENTRIES_EXTENDED_MAX)
 		offset = ALIGN(INITRD_START + INITRD_SIZE, sizeof(u64));