crypt_activate_by_keyslot_context never respected pbkdf serialation
flag (CRYPT_ACTIVATE_SERIALIZE_MEMORY_HARD_PBKDF).
In fact it worked only when device was activated via passphrase or via
passphrase file. It was never respected when device was activated
by a token for example.
When the internal code was fully switched to activation via keyslot
context the legacy code for passphrase based activation was dropped
and we lost track of serialization flag completely.
This fixes all of the issues so now the serialization flag will be
respected also with tokens (and all other activation methods unlocking
LUKS2 keyslot with memory hard pbkdf).
Fixes: 58385d68d8 (Allow activation via keyslot context)
Fixes: #968.
5d69c34f59