authenticate_2nd_new: only lock tfa config for recovery keys

authenticate_2nd_new: only lock tfa config for recovery keys

we currently only need to lock the tfa config when we got a recovery key
as a tfa challenge response, since that's the only thing that can
actually change the tfa config (every other method only reads from
there).

so to do that, factor out the code that was inside the lock, and call it
with/without lock depending on the tfa challenge response

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>