ovpn: consolidate crypto allocations in one chunk

ovpn: consolidate crypto allocations in one chunk

Currently ovpn uses three separate dynamically allocated structures to
set up cryptographic operations for both encryption and decryption. This
adds overhead to performance-critical paths and contribute to memory
fragmentation.

This commit consolidates those allocations into a single temporary blob,
similar to what esp_alloc_tmp() does.

The resulting performance gain is +7.7% and +4.3% for UDP when using AES
and ChaChaPoly respectively, and +4.3% for TCP.

Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>