Suppress error log when timerslack_ns write fails for already dead process

lmkd uses set_sched_priority to expedite killing of its victims and this
sometimes results in timerslack_ns writes for an process which is already
dead. These messages are detrimental during troubleshooting and do not
indicate a real problem, therefore need to be suppressed.

Bug: 124324891
Test: lmkd_unit_test

Change-Id: I5bc071b47d386c279dd9a56eee51dba7797f4520
Signed-off-by: Suren Baghdasaryan <surenb@google.com>