pam_fscrypt: filter out irrelevant policies earlier

pam_fscrypt: filter out irrelevant policies earlier

If a session is opened for a user twice and the second doesn't have the
AUTHTOK data, pam_fscrypt prints an error message that says it failed to
unlock a protector because AUTHTOK data is missing.  This is misleading
because the protector and its associated policies were already unlocked
by the first session.

To avoid this, move the check for whether the policy is provisioned or
not into policiesUsingProtector().  Also do the same for CloseSession.