crypto: jitter - output full sample from test interface

The Jitter RNG time delta is computed based on the difference of two
high-resolution, 64-bit time stamps. However, the test interface added
in 69f1c387 only outputs the lower 32 bits of those time stamps. To
ensure all information is available during the evaluation process of
the Jitter RNG, output the full 64-bit time stamps.

Any clients collecting data from the test interface will need to be
updated to take this change into account.

Additionally, the size of the temporary buffer that holds the data for
user space has been clarified. Previously, this buffer was
JENT_TEST_RINGBUFFER_SIZE (= 1000) bytes in size, however that value
represents the number of samples held in the kernel space ring buffer,
with each sample taking 8 (previously 4) bytes.

Rather than increasing the size to allow for all 1000 samples to be
output, we keep it at 1000 bytes, but clarify that this means at most
125 64-bit samples will be output every time this interface is called.

Reviewed-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Joachim Vandersmissen <git@jvdsn.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>