srcu: Make FORCE_NEED_SRCU_NMI_SAFE depend on RCU_EXPERT

The FORCE_NEED_SRCU_NMI_SAFE is useful only for those wishing to test
the SRCU code paths that accommodate architectures that do not have
NMI-safe per-CPU operations, that is, those architectures that do not
select the ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option.  As such, this
is a specialized Kconfig option that is not intended for casual users.

This commit therefore hides it behind the RCU_EXPERT Kconfig option.
Given that this new FORCE_NEED_SRCU_NMI_SAFE Kconfig option has no effect
unless the ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option is also selected,
it also depends on this Kconfig option.

[ paulmck: Apply Geert Uytterhoeven feedback. ]

[ boqun: Add the "Fixes" tag. ]

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Closes: https://lore.kernel.org/all/CAMuHMdX6dy9_tmpLkpcnGzxyRbe6qSWYukcPp=H1GzZdyd3qBQ@mail.gmail.com/
Fixes: 536e8b9 ("srcu: Add FORCE_NEED_SRCU_NMI_SAFE Kconfig for testing")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Author: paulmckrcu

kernel/rcu/Kconfig

@@ -68,6 +68,8 @@ config TREE_SRCU
 config FORCE_NEED_SRCU_NMI_SAFE
 	bool "Force selection of NEED_SRCU_NMI_SAFE"
 	depends on !TINY_SRCU
+	depends on RCU_EXPERT
+	depends on ARCH_HAS_NMI_SAFE_THIS_CPU_OPS
 	select NEED_SRCU_NMI_SAFE
 	default n
 	help