CRYPTO: Save prefix in s3crypt_sha512
Since commit 2951a9a84bd85f384213a3e071ffc167907df2d7 where we switched from
stpncpy to memcpy the salt prefix "$6$" is not stored at all.
This broke offline authentication if someone upgraded from old version
that stored the prefix to one that doesn't store it.
Resolves:
https://pagure.io/SSSD/sssd/issue/3791
Merges: https://pagure.io/SSSD/sssd/pull-request/3792
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>