torture: Avoid modulo-zero error in torture_hrtimeout_ns()

torture: Avoid modulo-zero error in torture_hrtimeout_ns()

Currently, all calls to torture_hrtimeout_ns() either provide a non-zero
fuzzt_ns or a NULL trsp, either of which avoids taking the modulus of a
zero-valued fuzzt_ns.  But this code should do a better job of defending
itself, so this commit explicitly checks fuzzt_ns and avoids the modulus
when its value is zero.

Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>