KEYS: trusted: Avoid calling null function trusted_key_exit

If one loads and unloads the trusted module, trusted_key_exit can be
NULL. Call it through static_call_cond() to avoid a kernel trap.

Fixes: 5d0682be ("KEYS: trusted: Add generic trusted keys framework")
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Cc: Sumit Garg <sumit.garg@linaro.org>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: Mimi Zohar <zohar@linux.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-integrity@vger.kernel.org
Cc: keyrings@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>