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_reserved(handle_t *handle, unsigned int line,
					int type)
{
	struct super_block *sb;
	int err;

	if (!ext4_handle_valid(handle))
		return ext4_get_nojournal();

	sb = handle->h_journal->j_private;
	trace_ext4_journal_start_reserved(sb, handle->h_buffer_credits,
					  _RET_IP_);
	err = ext4_journal_check_start(sb);
	if (err < 0) {
		jbd2_journal_free_reserved(handle);
		return ERR_PTR(err);
	}

	err = jbd2_journal_start_reserved(handle, type, line);
	if (err < 0)
		return ERR_PTR(err);
	return handle;
}

handle_t *__ext4_journal_start_reserved(handle_t *handle, unsigned int line,
					int type)
{
	struct super_block *sb;
	int err;

	if (!ext4_handle_valid(handle))
		return ext4_get_nojournal();

	sb = handle->h_journal->j_private;
	trace_ext4_journal_start_reserved(sb, handle->h_buffer_credits,
					  _RET_IP_);
	err = ext4_journal_check_start(sb);
	if (err < 0) {
		jbd2_journal_free_reserved(handle);
		return ERR_PTR(err);
	}

	err = jbd2_journal_start_reserved(handle, type, line);
	if (err < 0)
		return ERR_PTR(err);
	return handle;
}
