Fix activation of LUKS2 with capi format cipher and kernel crypt name.
While activation of internal cipher algorithms (like aes-generic) is disallowed, some old LUKS2 images can still use it.
Check the cipher in activate call, but allow to load LUKS2 metadata. This can allow to add repair code easily and also allow luksDump.
Also fix segfault in reencrypt code for such a header.
Fixes: #820 (closed)