random_get_pseudo_bytes() need not provide cryptographic strength entropy

Perf profiling of dd on a zvol revealed that my system spent 3.16% of
its time in random_get_pseudo_bytes(). No SPL consumers need
cryptographic strength entropy, so we can reduce our overhead by
changing the implementation to utilize a fast PRNG.

The Linux kernel did not export a suitable PRNG function until it
exported get_random_int() in Linux 3.10. While we could implement an
autotools check so that we use it when it is available or even try to
access the symbol on older kernels where it is not exported using the
fact that it is exported on newer ones as justification, we can instead
implement our own pseudo-random data generator. For this purpose, I have
written one based on a pseudo-random number generator proposed in a
paper by Sebastiano Vigna that itself was based on work by the late
George Marsaglia.

http://arxiv.org/pdf/1402.6246v2.pdf

Profiling the same benchmark with a variant of this patch that did not
disable interrupts showed that time spent in random_get_pseudo_bytes()
dropped to 0.06%. That is a factor of 50 improvement.

Signed-off-by: Richard Yao <ryao@gentoo.org>