netconsole: Use kstrtobool() instead of kstrtoint()

Replace kstrtoint() by kstrtobool() in the sysfs _store() functions.
This improves the user usability and simplify the code.

With this fix, it is now possible to use [YyNn] to set and unset a
feature. Old behaviour is still unchanged.

kstrtobool() is also safer and doesn't need the extra validation that
is required when converting a string to bool (end field in the struct),
which makes the code simpler.

Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230721092146.4036622-2-leitao@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>