srcu: Expedite srcu_schedule_cbs_snp() callback invocation

Although Tree SRCU does reduce delays when there is at least one
synchronize_srcu_expedited() invocation pending, srcu_schedule_cbs_snp()
still waits for SRCU_INTERVAL before invoking callbacks.  Since
synchronize_srcu_expedited() now posts a callback and waits for
that callback to do a wakeup, this destroys the expedited nature of
synchronize_srcu_expedited().  This destruction became apparent to
Marc Zyngier in the guise of a guest-OS bootup slowdown from five
seconds to no fewer than forty seconds.

This commit therefore invokes callbacks immediately at the end of the
grace period when there is at least one synchronize_srcu_expedited()
invocation pending.  This brought Marc's guest-OS bootup times back
into the realm of reason.

Reported-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Marc Zyngier <marc.zyngier@arm.com>