ext4: simplify the code of mb_find_order_for_block

The code of mb_find_order_for_block is a bit obscure, but we can
simplify it with mb_find_buddy(), make the code more concise.
Signed-off-by: Chunguang Xu <brookxu@tencent.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Link: https://lore.kernel.org/r/1604764698-4269-3-git-send-email-brookxu@tencent.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -1289,22 +1289,18 @@ static void ext4_mb_unload_buddy(struct ext4_buddy *e4b)
 
 static int mb_find_order_for_block(struct ext4_buddy *e4b, int block)
 {
-	int order = 1;
-	int bb_incr = 1 << (e4b->bd_blkbits - 1);
+	int order = 1, max;
 	void *bb;
 
 	BUG_ON(e4b->bd_bitmap == e4b->bd_buddy);
 	BUG_ON(block >= (1 << (e4b->bd_blkbits + 3)));
 
-	bb = e4b->bd_buddy;
 	while (order <= e4b->bd_blkbits + 1) {
-		block = block >> 1;
-		if (!mb_test_bit(block, bb)) {
+		bb = mb_find_buddy(e4b, order, &max);
+		if (!mb_test_bit(block >> order, bb)) {
 			/* this block is part of buddy of order 'order' */
 			return order;
 		}
-		bb += bb_incr;
-		bb_incr >>= 1;
 		order++;
 	}
 	return 0;