libceph: introduce ceph_crypto_key_prepare()

libceph: introduce ceph_crypto_key_prepare()

In preparation for bringing in a new encryption scheme/key type,
decouple decoding or cloning the key from allocating required crypto
API objects and setting them up.  The rationale is that a) in some
cases a shallow clone is sufficient and b) ceph_crypto_key_prepare()
may grow additional parameters that would be inconvenient to provide
at the point the key is originally decoded.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>