Changes

bpftrace: Introduce ksys_write_delay.bt

This will record latency of write() calls during ttcn3-bts-test run,
where some write() call on stderr seems to be taking ~0.32 seconds,
stalling the timerfd (71 times) and making osmo-bts exit.

Related: OS#6794
Change-Id: Ia1d2036ac7c6c9e2818d07e8787d857b3b3e5881