block: use FIELD_SIZEOF to calculate size of a field

use FIELD_SIZEOF instead of open coding
Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-core.c

@@ -3370,7 +3370,7 @@ EXPORT_SYMBOL(blk_post_runtime_resume);
 int __init blk_dev_init(void)
 {
 	BUILD_BUG_ON(__REQ_NR_BITS > 8 *
-			sizeof(((struct request *)0)->cmd_flags));
+			FIELD_SIZEOF(struct request, cmd_flags));
 
 	/* used for unplugging and affects IO latency/throughput - HIGHPRI */
 	kblockd_workqueue = alloc_workqueue("kblockd",