random: adjust the generator polynomials in the mixing function slightly

Our mixing functions were analyzed by Lacharme, Roeck, Strubel, and
Videau in their paper, "The Linux Pseudorandom Number Generator
Revisited" (see: http://eprint.iacr.org/2012/251.pdf).

They suggested a slight change to improve our mixing functions
slightly.  I also adjusted the comments to better explain what is
going on, and to document why the polynomials were changed.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>