random: allow fractional bits to be tracked

Allow fractional bits of entropy to be tracked by scaling the entropy
counter (fixed point).  This will be used in a subsequent patch that
accounts for entropy lost due to overwrites.

[ Modified by tytso to fix up a few missing places where the
  entropy_count wasn't properly converted from fractional bits to
  bits. ]
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>