clear_warn_once: bind a timer to written reset value

Existing documentation has a write of "1" to clear/reset all the
WARN_ONCE and similar to the as-booted state, so they can possibly
be re-triggered again during debugging/testing.

But having them auto-reset once a day, or once a week, might shed
valuable information to a sysadmin on what the system is doing while
trying to debug an issue.

Here we extend the existing debugfs variable to bind a timer to the
written value N, so that it will reset every N minutes, for N>1.
Writing a zero will clear any previously set timer value.

The pre-existing behaviour of writing N=1 will do a one-shot clear.

Link: https://lore.kernel.org/r/20201126063029.2030-1-paul.gortmaker@windriver.com
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>