apparmor: Fix memleak in alloc_ns()

After changes in commit a1bd627b ("apparmor: share profile name on
replacement"), the hname member of struct aa_policy is not valid slab
object, but a subset of that, it can not be freed by kfree_sensitive(),
use aa_policy_destroy() to fix it.

Fixes: a1bd627b ("apparmor: share profile name on replacement")
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>