The output n bits can receive more than n bits of min entropy, of course,
but the fixed output of the conditioning function can only asymptotically
approach the output size bits of min entropy, not attain that bound.
Random maps will tend to have output collisions, which reduces the
creditable output entropy (that is what SP 800-90B Section 3.1.5.1.2
attempts to bound).

The value "64" is justified in Appendix A.4 of the current 90C draft,
and aligns with NIST's in "epsilon" definition in this document, which is
that a string can be considered "full entropy" if you can bound the min
entropy in each bit of output to at least 1-epsilon, where epsilon is
required to be <= 2^(-32).

Note, this patch causes the Jitter RNG to cut its performance in half in
FIPS mode because the conditioning function of the LFSR produces 64 bits
of entropy in one block. The oversampling requires that additionally 64
bits of entropy are sampled from the noise source. If the conditioner is
changed, such as using SHA-256, the impact of the oversampling is only
one fourth, because for the 256 bit block of the conditioner, only 64
additional bits from the noise source must be sampled.

This patch is derived from the user space jitterentropy-library.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Reviewed-by: Simo Sorce <simo@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>