staging: lustre: Replace printk_ratelimited with pr_warn_ratelimited

This patch fixes the checkpoint.pl warning:

WARNING: Prefer printk_ratelimited or pr_<level>_ratelimited to
printk_ratelimit

Signed-off-by: Konrad Malkowski <konrad.malkowski@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>