random: make /dev/urandom scalable for silly userspace programs

On a system with a 4 socket (NUMA) system where a large number of
application threads were all trying to read from /dev/urandom, this
can result in the system spending 80% of its time contending on the
global urandom spinlock.  The application should have used its own
PRNG, but let's try to help it from running, lemming-like, straight
over the locking cliff.

Reported-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>