srcu: Prohibit call_srcu() use under raw spinlocks

Invoking queue_delayed_work() while holding a raw spinlock is forbidden
in -rt kernels, which is exactly what __call_srcu() does, indirectly via
srcu_funnel_gp_start().  This commit therefore downgrades Tree SRCU's
locking from raw to non-raw spinlocks, which works because call_srcu()
is not ever called while holding a raw spinlock.

Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>