clear_warn_once: add a clear_warn_once= boot parameter

In the event debugfs is not mounted, or if built with the .config
setting DEBUG_FS_ALLOW_NONE chosen, this gives the sysadmin access
to reset the WARN_ONCE() state on a periodic basis.

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>