selftests: bpf: crypto: adjust bench to use nullable IV

selftests: bpf: crypto: adjust bench to use nullable IV

The bench shows some improvements, around 4% faster on decrypt.

Before:

Benchmark 'crypto-decrypt' started.
Iter   0 (325.719us): hits    5.105M/s (  5.105M/prod), drops 0.000M/s, total operations    5.105M/s
Iter   1 (-17.295us): hits    5.224M/s (  5.224M/prod), drops 0.000M/s, total operations    5.224M/s
Iter   2 (  5.504us): hits    4.630M/s (  4.630M/prod), drops 0.000M/s, total operations    4.630M/s
Iter   3 (  9.239us): hits    5.148M/s (  5.148M/prod), drops 0.000M/s, total operations    5.148M/s
Iter   4 ( 37.885us): hits    5.198M/s (  5.198M/prod), drops 0.000M/s, total operations    5.198M/s
Iter   5 (-53.282us): hits    5.167M/s (  5.167M/prod), drops 0.000M/s, total operations    5.167M/s
Iter   6 (-17.809us): hits    5.186M/s (  5.186M/prod), drops 0.000M/s, total operations    5.186M/s
Summary: hits    5.092 ± 0.228M/s (  5.092M/prod), drops    0.000 ±0.000M/s, total operations    5.092 ± 0.228M/s

After:

Benchmark 'crypto-decrypt' started.
Iter   0 (268.912us): hits    5.312M/s (  5.312M/prod), drops 0.000M/s, total operations    5.312M/s
Iter   1 (124.869us): hits    5.354M/s (  5.354M/prod), drops 0.000M/s, total operations    5.354M/s
Iter   2 (-36.801us): hits    5.334M/s (  5.334M/prod), drops 0.000M/s, total operations    5.334M/s
Iter   3 (254.628us): hits    5.334M/s (  5.334M/prod), drops 0.000M/s, total operations    5.334M/s
Iter   4 (-77.691us): hits    5.275M/s (  5.275M/prod), drops 0.000M/s, total operations    5.275M/s
Iter   5 (-164.510us): hits    5.313M/s (  5.313M/prod), drops 0.000M/s, total operations    5.313M/s
Iter   6 (-81.376us): hits    5.346M/s (  5.346M/prod), drops 0.000M/s, total operations    5.346M/s
Summary: hits    5.326 ± 0.029M/s (  5.326M/prod), drops    0.000 ±0.000M/s, total operations    5.326 ± 0.029M/s

Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Vadim Fedorenko <vadfed@meta.com>
Link: https://lore.kernel.org/r/20240613211817.1551967-5-vadfed@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>