fix timespec subtraction in compute_elapsed_us()
fix timespec subtraction in compute_elapsed_us()

The previous implementation unconditionally subtracted nanosecond
values from different time measurements, causing overflow if the
current measurement was taken in less of a fraction of a second
than the past measurement. Use timespecsub() instead, which
accounts for nanoseconds correctly.
This is a similar bug as fixed in osmo-pcu for issue OS#3225

While here, switch variables which are calculated based on
struct timespec to 64 bit types. While probably not strictly necessary
in practice, this makes the types used in calculations more compatible.

Change-Id: Idfd9c807e35cd7fb5c80625b9746121f81c24599
Related: OS#3467
Related: OS#3225

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