printk: Monitor change of console loglevel.

This patch detects change of console loglevel in case some testcase by
error changed console loglevel [1]. This patch is intended for testing
on linux-next.git only, and will be removed after confirming that this
patch finds nothing.

[1] https://lore.kernel.org/lkml/4d1a4b51-999b-63c6-5ce3-a704013cecb6@i-love.sakura.ne.jp/



Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>