pstore: Expose kmsg_bytes as a module parameter

Currently this tuning is only exposed as a filesystem option,
but most Linux distros automatically mount pstore, hence changing
this setting requires remounting it. Also, if that mount option
wasn't explicitly set it doesn't show up in mount information,
so users cannot check what is the current value of kmsg_bytes.

Let's then expose it as a module parameter, allowing both user
visibility at all times (even if not manually set) and also the
possibility of setting that as a boot/module parameter.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221006224212.569555-3-gpiccoli@igalia.com