use oom_score_adj instead of oom_adj for avoiding OOM killer

Using /proc/self/oom_adj for avoiding OOM killer of linux is
deprecated because it will be removed in the future. This patch lets
tgtd use /proc/self/oom_score_adj if it is available.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>