client: Do not expose ceph_fscrypt_key_identifier in api

client: Do not expose ceph_fscrypt_key_identifier in api

The libcephfs API call add_fscrypt_key exposes an internal fscrypt
data structure. This is because a hash keyid (of the master key) is used
for calls such as remove_fscrypt_key. Instead of using this structure,
use a char array to obtain keyid.

Fixes: https://tracker.ceph.com/issues/63293
Signed-off-by: Christopher Hoffman <choffman@redhat.com>