ext4: use common helpers in all places reading metadata buffers

Revome all open codes that read metadata buffers, switch to use
ext4_read_bh_*() common helpers.

Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Suggested-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20200924073337.861472-4-yi.zhang@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
index 8e7e971..dea738b 100644
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -494,12 +494,10 @@ ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group,
 	 * submit the buffer_head for reading
 	 */
 	set_buffer_new(bh);
-	clear_buffer_verified(bh);
 	trace_ext4_read_block_bitmap_load(sb, block_group, ignore_locked);
-	bh->b_end_io = ext4_end_bitmap_read;
-	get_bh(bh);
-	submit_bh(REQ_OP_READ, REQ_META | REQ_PRIO |
-		  (ignore_locked ? REQ_RAHEAD : 0), bh);
+	ext4_read_bh_nowait(bh, REQ_META | REQ_PRIO |
+			    (ignore_locked ? REQ_RAHEAD : 0),
+			    ext4_end_bitmap_read);
 	return bh;
 verify:
 	err = ext4_validate_block_bitmap(sb, desc, block_group, bh);