netconsole: Use sysfs_emit() instead of snprintf()

According to the sysfs.rst documentation, _show() functions should only
use sysfs_emit() instead of snprintf().

Since snprintf() shouldn't be used in the sysfs _show() path, replace it
by sysfs_emit().

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/20230721092146.4036622-1-leitao@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>