rcu: Eliminate check_cpu_stall() duplicate code

The code and comments of self-detected and other-detected RCU CPU stall
warnings are identical except the output function.  This commit therefore
refactors so as to consolidate the duplicate code.

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