ext4: fix potential use after free in __ext4_journal_stop

There is a use-after-free possibility in __ext4_journal_stop() in the
case that we free the handle in the first jbd2_journal_stop() because
we're referencing handle->h_err afterwards. This was introduced in
9705acd63b125dee8b15c705216d7186daea4625 and it is wrong. Fix it by
storing the handle->h_err value beforehand and avoid referencing
potentially freed handle.

Fixes: 9705acd63b125dee8b15c705216d7186daea4625
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Cc: stable@vger.kernel.org
handle_t *__ext4_journal_start_sb(struct super_block *sb, unsigned int line,
				  int type, int blocks, int rsv_blocks)
{
	journal_t *journal;
	int err;

	trace_ext4_journal_start(sb, blocks, rsv_blocks, _RET_IP_);
	err = ext4_journal_check_start(sb);
	if (err < 0)
		return ERR_PTR(err);

	journal = EXT4_SB(sb)->s_journal;
	if (!journal)
		return ext4_get_nojournal();
	return jbd2__journal_start(journal, blocks, rsv_blocks, GFP_NOFS,
				   type, line);
}

handle_t *__ext4_journal_start_sb(struct super_block *sb, unsigned int line,
				  int type, int blocks, int rsv_blocks)
{
	journal_t *journal;
	int err;

	trace_ext4_journal_start(sb, blocks, rsv_blocks, _RET_IP_);
	err = ext4_journal_check_start(sb);
	if (err < 0)
		return ERR_PTR(err);

	journal = EXT4_SB(sb)->s_journal;
	if (!journal)
		return ext4_get_nojournal();
	return jbd2__journal_start(journal, blocks, rsv_blocks, GFP_NOFS,
				   type, line);
}
