rcu: Do not NMI offline CPUs

mainline inclusion
from mainline-v5.12-rc1
commit 725969ac
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7OIXK

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=725969ac11d7fa50aa701321daa600ce421fc21b



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Currently, RCU CPU stall warning messages will NMI whatever CPU looks
like it is blocking either the current grace period or the grace-period
kthread.  This can produce confusing output if the target CPU is offline.
This commit therefore checks for offline CPUs.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>