rcutorture: Enable SRCU readers from timer handler

Now that it is legal to invoke srcu_read_lock() and srcu_read_unlock()
for a given srcu_struct from both process context and {soft,}irq
handlers, it is time to test it.  This commit therefore enables
testing of SRCU readers from rcutorture's timer handler, using in_task()
to determine whether or not it is safe to sleep in the SRCU read-side
critical sections.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>