geli: Fix geli setkey -J for detached providers

Clear cached_passphrase before generating a new key, otherwise the
operation nonsensically tries to reuse the old passphrase.

Approved by: re (kib)
PR: 254966
Pull Request: https://github.com/freebsd/freebsd-src/pull/780
MFC after: 1 week

(cherry picked from commit 88d13bf33754bd4b0c5df92eef83d6fadb9b4944)