rngd: Allow up to a 1:1000 false error rate on FIPS tests

rngd: Allow up to a 1:1000 false error rate on FIPS tests

The FIPS tests have a measured false positive error rate of
approximately 1:1250.  In order to not permanently disable a
functioning random number source under high traffic, allow
one failure per 1000 successful blocks.

However, never allow more than 25 subsequent failures; this is
handled by not allowing the failures counter to go below zero.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>