crypto: jitterentropy - use ktime_get_ns as fallback

As part of the Y2038 development, __getnstimeofday is not supposed to be
used any more. It is now replaced with ktime_get_ns. The Jitter RNG uses
the time stamp to measure the execution time of a given code path and
tries to detect variations in the execution time. Therefore, the only
requirement the Jitter RNG has, is a sufficient high resolution to
detect these variations.

The change was tested on x86 to show an identical behavior as RDTSC. The
used test code simply measures the execution time of the heart of the
RNG:

        jent_get_nstime(&time);
        jent_memaccess(ec, min);
        jent_fold_time(NULL, time, &folded, min);
        jent_get_nstime(&time2);
        return ((time2 - time));

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>