This fixes crypt_keyslot_add_by_key where we were unable to store
keyslot (unbound to segment) with different key_size.
The code used (new) volume key size implicitly which could be wrong
if new size was not compatible with cipher parameter for keyslot area.
Remove code for handling multiple digests per single keyslot.
Same would apply to segments with the only exception of segment
in-reencryption. We need that exception so that we will not lose
old key digests too early.
