jbd2: fix potential use-after-free in jbd2_fc_wait_bufs

In 'jbd2_fc_wait_bufs' use 'bh' after put buffer head reference count
which may lead to use-after-free.
So judge buffer if uptodate before put buffer head reference count.

Cc: stable@kernel.org
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220914100812.1414768-3-yebin10@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/jbd2/journal.c

@@ -923,16 +923,16 @@ int jbd2_fc_wait_bufs(journal_t *journal, int num_blks)
 	for (i = j_fc_off - 1; i >= j_fc_off - num_blks; i--) {
 		bh = journal->j_fc_wbuf[i];
 		wait_on_buffer(bh);
-		put_bh(bh);
-		journal->j_fc_wbuf[i] = NULL;
 		/*
 		 * Update j_fc_off so jbd2_fc_release_bufs can release remain
 		 * buffer head.
 		 */
 		if (unlikely(!buffer_uptodate(bh))) {
-			journal->j_fc_off = i;
+			journal->j_fc_off = i + 1;
 			return -EIO;
 		}
+		put_bh(bh);
+		journal->j_fc_wbuf[i] = NULL;
 	}
 
 	return 0;