linprocfs: Add /proc/self/oom_score_adj.

To avoid annoyng messages from LTP test suites add the simple
implementation of /proc/self/oom_score_adj which is do nothing.

Reviewed by: emaste
Differential revision: https://reviews.freebsd.org/D34710
MFC after: 2 weeks

(cherry picked from commit b7df7b987e8fac05006aa5f132c424e2b2bcf156)