jbd2: flush_descriptor(): Do not decrease buffer head's ref count

When executing generic/388 on a ppc64le machine, we notice the following
call trace,

VFS: brelse: Trying to free free buffer
WARNING: CPU: 0 PID: 6637 at /root/repos/linux/fs/buffer.c:1195 __brelse+0x84/0xc0

Call Trace:
 __brelse+0x80/0xc0 (unreliable)
 invalidate_bh_lru+0x78/0xc0
 on_each_cpu_mask+0xa8/0x130
 on_each_cpu_cond_mask+0x130/0x170
 invalidate_bh_lrus+0x44/0x60
 invalidate_bdev+0x38/0x70
 ext4_put_super+0x294/0x560
 generic_shutdown_super+0xb0/0x170
 kill_block_super+0x38/0xb0
 deactivate_locked_super+0xa4/0xf0
 cleanup_mnt+0x164/0x1d0
 task_work_run+0x110/0x160
 do_notify_resume+0x414/0x460
 ret_from_except_lite+0x70/0x74

The warning happens because flush_descriptor() drops bh reference it
does not own. The bh reference acquired by
jbd2_journal_get_descriptor_buffer() is owned by the log_bufs list and
gets released when this list is processed. The reference for doing IO is
only acquired in write_dirty_buffer() later in flush_descriptor().

Reported-by: Harish Sriram <harish@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Chandan Rajendra <chandan@linux.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/jbd2/revoke.c

@@ -638,10 +638,8 @@ static void flush_descriptor(journal_t *journal,
 {
 	jbd2_journal_revoke_header_t *header;
 
-	if (is_journal_aborted(journal)) {
-		put_bh(descriptor);
+	if (is_journal_aborted(journal))
 		return;
-	}
 
 	header = (jbd2_journal_revoke_header_t *)descriptor->b_data;
 	header->r_count = cpu_to_be32(offset);