kernel/rcu/tree_stall: add /sys/kernel/rcu_stall_count

Expose a simple counter to userspace for monitoring tools.

Link: https://lkml.kernel.org/r/20250504180831.4190860-3-max.kellermann@ionos.com
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Cc: Core Minyard <cminyard@mvista.com>
Cc: Doug Anderson <dianders@chromium.org>
Cc: Joel Granados <joel.granados@kernel.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Song Liu <song@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: MaxKellermann

Documentation/ABI/testing/sysfs-kernel-rcu_stall_count

@@ -0,0 +1,6 @@
+What:		/sys/kernel/rcu_stall_count
+Date:		May 2025
+KernelVersion:	6.16
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:
+		Shows how many times the system has detected an RCU stall since last boot.