ocfs2: fix null-ptr-deref when journal load failed.

During the mounting process, if journal_reset() fails because of too short
journal, then lead to jbd2_journal_load() fails with NULL j_sb_buffer. 
Subsequently, ocfs2_journal_shutdown() calls
jbd2_journal_flush()->jbd2_cleanup_journal_tail()->
__jbd2_update_log_tail()->jbd2_journal_update_sb_log_tail()
->lock_buffer(journal->j_sb_buffer), resulting in a null-pointer
dereference error.

To resolve this issue, we should check the JBD2_LOADED flag to ensure the
journal was properly loaded.  Additionally, use journal instead of
osb->journal directly to simplify the code.

Link: https://syzkaller.appspot.com/bug?extid=05b9b39d8bdfe1a0861f
Link: https://lkml.kernel.org/r/[email protected]
Fixes: f6f50e2 ("jbd2: Fail to load a journal if it is too short")
Signed-off-by: Julian Sun <[email protected]>
Reported-by: [email protected]
Suggested-by: Joseph Qi <[email protected]>
Reviewed-by: Joseph Qi <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Junxiao Bi <[email protected]>
Cc: Changwei Ge <[email protected]>
Cc: Gang He <[email protected]>
Cc: Jun Piao <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: jc2870

fs/ocfs2/journal.c

@@ -1055,7 +1055,7 @@ void ocfs2_journal_shutdown(struct ocfs2_super *osb)
 	if (!igrab(inode))
 		BUG();
 
-	num_running_trans = atomic_read(&(osb->journal->j_num_trans));
+	num_running_trans = atomic_read(&(journal->j_num_trans));
 	trace_ocfs2_journal_shutdown(num_running_trans);
 
 	/* Do a commit_cache here. It will flush our journal, *and*
@@ -1074,9 +1074,10 @@ void ocfs2_journal_shutdown(struct ocfs2_super *osb)
 		osb->commit_task = NULL;
 	}
 
-	BUG_ON(atomic_read(&(osb->journal->j_num_trans)) != 0);
+	BUG_ON(atomic_read(&(journal->j_num_trans)) != 0);
 
-	if (ocfs2_mount_local(osb)) {
+	if (ocfs2_mount_local(osb) &&
+	    (journal->j_journal->j_flags & JBD2_LOADED)) {
 		jbd2_journal_lock_updates(journal->j_journal);
 		status = jbd2_journal_flush(journal->j_journal, 0);
 		jbd2_journal_unlock_updates(journal->j_journal);