jbd2: increase IO priority of checkpoint

In commit 6a3afb6 ("jbd2: increase the journal IO's priority"),
the priority of IOs initiated by jbd2 has been raised, exempting them
from WBT throttling.
Checkpoint is also a crucial operation of jbd2. While no serious issues
have been observed so far, it should still be reasonable to exempt
checkpoint from WBT throttling.

Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: Julian Sun

fs/jbd2/checkpoint.c

@@ -131,7 +131,7 @@ __flush_batch(journal_t *journal, int *batch_count)
 
 	blk_start_plug(&plug);
 	for (i = 0; i < *batch_count; i++)
-		write_dirty_buffer(journal->j_chkpt_bhs[i], REQ_SYNC);
+		write_dirty_buffer(journal->j_chkpt_bhs[i], JBD2_JOURNAL_REQ_FLAGS);
 	blk_finish_plug(&plug);
 
 	for (i = 0; i < *batch_count; i++) {