Fix test_looplen, use double-precition arithmetic

Here you go!!!

        [root@perfsqh userspace-rcu]# ./test_looplen
        CALIBRATION : 0 cycles per loop

Hmmm...  Probably not what you had in mind.  Breaking out the
floating-point arithmetic:

        [root@perfsqh userspace-rcu]# ./test_looplen
        CALIBRATION : 0.386172 cycles per loop
        time_tot = 4049312, LOOPS = 1048576, TESTS = 10

More helpful?  ;-)

                                                Thanx, Paul

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>