staging: lustre: ldlm: ldlm_resource.c replace obsolete function with kstrtoul()

- this change fixes the warning "WARNING: simple_strtoul is obsolete, use
kstrtoul instead"
- removed the unused var 'end' as this is no longer required for
  kstrtoul()

Signed-off-by: Anil Belur <askb23@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>