apparmor: fix a memleak in multi_transaction_new()

In multi_transaction_new(), the variable t is not freed or passed out
on the failure of copy_from_user(t->data, buf, size), which could lead
to a memleak.

Fix this bug by adding a put_multi_transaction(t) in the error path.

Fixes: 1dea3b41 ("apparmor: speed up transactional queries")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>