Callback invocation can run for a significant time period, and within
CONFIG_NO_HZ_FULL=y kernels, this period will be devoid of scheduler-clock
interrupts.  In-kernel execution without such interrupts can cause all
manner of malfunction, with RCU CPU stall warnings being but one result.

This commit therefore forces scheduling-clock interrupts on whenever more
than a few RCU callbacks are invoked.  Because offloaded callback invocation
can be preempted, this forcing is withdrawn on each context switch.  This
in turn requires that the loop invoking RCU callbacks reiterate the forcing
periodically.

[ paulmck: Apply Joel Fernandes TICK_DEP_MASK_RCU->TICK_DEP_BIT_RCU fix. ]
[ paulmck: Remove NO_HZ_FULL check per Frederic Weisbecker feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>