Journal too short will cause ocfs2_check_volume failed, and will set journal->j_sb_buffer to NULL

#syz test: upstream c3f2d783a459

diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
index 530fba34f6d3..25821077b855 100644
--- a/fs/ocfs2/journal.c
+++ b/fs/ocfs2/journal.c
@@ -1077,9 +1077,11 @@ void ocfs2_journal_shutdown(struct ocfs2_super *osb)
 	BUG_ON(atomic_read(&(osb->journal->j_num_trans)) != 0);
 
 	if (ocfs2_mount_local(osb)) {
-		jbd2_journal_lock_updates(journal->j_journal);
-		status = jbd2_journal_flush(journal->j_journal, 0);
-		jbd2_journal_unlock_updates(journal->j_journal);
+		if (journal->j_journal->j_sb_buffer) {
+			jbd2_journal_lock_updates(journal->j_journal);
+			status = jbd2_journal_flush(journal->j_journal, 0);
+			jbd2_journal_unlock_updates(journal->j_journal);
+		}
 		if (status < 0)
 			mlog_errno(status);
 	}

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+05b9b39d8bdfe1a0861f@syzkaller.appspotmail.com
Tested-by: syzbot+05b9b39d8bdfe1a0861f@syzkaller.appspotmail.com

Tested on:

commit:         c3f2d783 Merge tag 'mm-hotfixes-stable-2024-08-17-19-3..
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=14739d83980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=7229118d88b4a71b
dashboard link: https://syzkaller.appspot.com/bug?extid=05b9b39d8bdfe1a0861f
compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
patch:          https://syzkaller.appspot.com/x/patch.diff?x=14ef258d980000

Note: testing is done by a robot and is best-effort only.