cleanerd: adjust the OOM killer

cleanerd is a very important process in a system which uses nilfs. So
it should adjust the OOM killer for reducing possibility of the
killing as much as possible.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>