staging: lustre: replace cfs_rand() with prandom_u32_max()

All occurrences of
   cfs_rand() % X
are replaced with
   prandom_u32_max(X)

cfs_rand() is a simple Linear Congruential PRNG.  prandom_u32_max()
is at least as random, is seeded with more randomness, and uses
cpu-local state to avoid cross-cpu issues.

This is the first step is discarding the libcfs prng with
the standard linux prng.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>