bts-trx: reorder first timerfd schedule to decrease first timeout skew
bts-trx: reorder first timerfd schedule to decrease first timeout skew

By reordering the instruction, we scheduler the timerfd prior to
processing the FN on the upper layers. This means the first timerfd
expiration even will happen more inline with the expected time, that is,
CLOCK IND time + GSM_TDMA_FN_DURATION_nS.

Let T(trx_sched_fn) be the time spent executing function trx_sched_fn().
With previous order, the timerfd would have been scheduled later, which
in the end would mean expiration would happen at time CLOCK_IND +
GSM_TDMA_FN_DURATION_nS + T(trx_sched_fn), hence ending up with an extra
skew of T(trx_sched_fn) added by ourselves.
This extra skew added may be important specially at startup time (when
this code path is used), since usually the load in the system is high
and skew is usually already higher, which means helping crossing
unacceptable thresholds which may end up in osmo-bts-trx stopping with
"No clock from osmo-trx" reason.

Change-Id: Ie2ba35cd87f0bd4078ac3b4b5ec2eacad36c4258

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts