if_ovpn: remove incorrect rounding up of packet sizes

The ciphers used by OpenVPN (DCO) do not require data to be block-sized.
Do not round up to AES_BLOCK_LEN, as this can lead to issues with
fragmented packets.

Reported by: Gert Doering <gert@greenie.muc.de>
Sponsored by: Rubicon Communications, LLC ("Netgate")