devcoredump: provide a one-way disable function

Since device/firmware coredumps can contain private data, it can
be desirable to turn them off unconditionally to be certain that
no such data will be collected by the system.

To achieve this, provide a "disabled" sysfs class attribute that
can only be changed from 0 to 1 and not back. Upon disabling,
discard existing coredumps and stop storing new ones.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>