l1sap: Fix calculation of expired RACH slots in case of missing frame numbers
l1sap: Fix calculation of expired RACH slots in case of missing frame numbers

In case of a Combined CCCH (with SDCCH/4), the number of RACH slots
depends on the frame number.  So in case of lost/skipped frame numbers,
we cannot simply compute the value for the current fn and then multiply
it by the number of frame numbers expired.  Rather, we have to 'replay'
all missed frame numbers and individually determine how many RACH
slots happened in that frame.

Related: OS#3750
Change-Id: If4f8d2ea55fc722c64c330cde09e833b67ee98fe

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