rcu/rcutorture: Replace 0 with false

Coccinelle reports a warning

WARNING: Assignment of 0/1 to bool variable

The root cause is that the variable lastphase is a bool, but is
initialised with integer 0.  This commit therefore replaces the 0 with
a false.

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>