gostr3411-94: small speedup

* cipher/gostr3411-94.c (do_p): unroll loop for a small spedup

Before:
GOSTR3411_94 | 25.12 ns/B 37.96 MiB/s - c/B
GOSTR3411_CP | 25.14 ns/B 37.93 MiB/s - c/B
After:
GOSTR3411_94 | 24.57 ns/B 38.81 MiB/s - c/B
GOSTR3411_CP | 24.59 ns/B 38.79 MiB/s - c/B

• Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>