rcu: Make the TASKS_RCU Kconfig option be selected

Currently, any kernel built with CONFIG_PREEMPTION=y also gets
CONFIG_TASKS_RCU=y, which is not helpful to people trying to build
preemptible kernels of minimal size.

Because CONFIG_TASKS_RCU=y is needed only in kernels doing tracing of
one form or another, this commit moves from TASKS_RCU deciding when it
should be enabled to the tracing Kconfig options explicitly selecting it.
This allows building preemptible kernels without TASKS_RCU, if desired.

This commit also updates the SRCU-N and TREE09 rcutorture scenarios
in order to avoid Kconfig errors that would otherwise result from
CONFIG_TASKS_RCU being selected without its CONFIG_RCU_EXPERT dependency
being met.

[ paulmck: Apply BPF_SYSCALL feedback from Andrii Nakryiko. ]

Reported-by: Hyeonggon Yoo <[email protected]>
Tested-by: Hyeonggon Yoo <[email protected]>
Tested-by: Zhouyi Zhou <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Acked-by: Masami Hiramatsu <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

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