random: fix error in entropy extraction (CVE-2007-2453 1 of 2)

Fix cast error in entropy extraction.
Add comments explaining the magic 16.
Remove extra confusing loop variable.

Signed-off-by: Matt Mackall <mpm@selenic.com>
Acked-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Adrian Bunk <bunk@kernel.org>