libfido2-util: accept cached pin in fido2_generate_hmac_hash()

libfido2-util: accept cached pin in fido2_generate_hmac_hash()

fido2_generate_hmac_hash() sets req->keyring to "fido2-pin" when
calling ask_password_auto(), suggesting that a key by this name
can be read from the kernel keyring. But the keyring is never
opened because the ASK_PASSWORD_ACCEPT_CACHED flag is not set.

Set ASK_PASSWORD_ACCEPT_CACHED to allow automated / scripted
setup of encrypted volumes with FIDO2. If the PIN turns out to
be invalid, clear ASK_PASSWORD_ACCEPT_CACHED to avoid retrying
and possible lockout.