rcutorture: Correctly compute probability to invoke ->exp_current()

rcutorture: Correctly compute probability to invoke ->exp_current()

Lack of parentheses causes the ->exp_current() function, for example,
srcu_expedite_current(), to be called only once in four billion times
instead of the intended once in 256 times.  This commit therefore adds
the needed parentheses.

Reported-by: Chris Mason <clm@meta.com>
Reported-by: Joel Fernandes <joelagnelf@nvidia.com>
Fixes: 950063c6e897 ("rcutorture: Test srcu_expedite_current()")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>