ext4: fix incorrect tid assumption in jbd2_journal_shrink_checkpoint_list()

Function jbd2_journal_shrink_checkpoint_list() assumes that '0' is not a
valid value for transaction IDs, which is incorrect.  Don't assume that and
use two extra boolean variables to control the loop iterations and keep
track of the first and last tid.
Signed-off-by: Luis Henriques (SUSE) <luis.henriques@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240724161119.13448-4-luis.henriques@linux.devSigned-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/jbd2/checkpoint.c

@@ -413,6 +413,7 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
 	tid_t tid = 0;
 	unsigned long nr_freed = 0;
 	unsigned long freed;
+	bool first_set = false;
 
 again:
 	spin_lock(&journal->j_list_lock);
@@ -432,8 +433,10 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
 	else
 		transaction = journal->j_checkpoint_transactions;
 
-	if (!first_tid)
+	if (!first_set) {
 		first_tid = transaction->t_tid;
+		first_set = true;
+	}
 	last_transaction = journal->j_checkpoint_transactions->t_cpprev;
 	next_transaction = transaction;
 	last_tid = last_transaction->t_tid;
@@ -463,7 +466,7 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
 	spin_unlock(&journal->j_list_lock);
 	cond_resched();
 
-	if (*nr_to_scan && next_tid)
+	if (*nr_to_scan && journal->j_shrink_transaction)
 		goto again;
 out:
 	trace_jbd2_shrink_checkpoint_list(journal, first_tid, tid, last_tid,