clear_warn_once: expand debugfs to include read support

The existing clear_warn_once variable is write-only; used as per the
documentation to reset the warn_once to as-booted state with:

	echo 1 > /sys/kernel/debug/clear_warn_once

The objective is to expand on that functionality, which requires the
debugfs variable to be read/write and not just write-only.

Here, we deal with the debugfs boilerplate associated with converting
it from write-only to read-write, in order to factor that out for easier
review, and for what may be a possible future useful bisect point.

Existing functionality is unchanged - the only difference is that we
have tied in a variable that lets you now read the variable and see
the last value written.

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>