rcu: Once again use NMI-based stack traces in stall warnings

This commit is for all intents and purposes a revert of bc1dce514e9b
("rcu: Don't use NMIs to dump other CPUs' stacks").  The reason to suppose
that this can now safely be reverted is the presence of 42a0bb3f7138
("printk/nmi: generic solution for safe printk in NMI"), which is said
to have made NMI-based stack dumps safe.

However, this reversion keeps one nice property of bc1dce514e9b
("rcu: Don't use NMIs to dump other CPUs' stacks"), namely that
only those CPUs blocking the grace period are dumped.  The new
trigger_single_cpu_backtrace() is used to make this happen, as
suggested by Josh Poimboeuf.

Reported-by: Vince Weaver <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Cc: Petr Mladek <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Reviewed-by: Josh Poimboeuf <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>

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