vss: Clean up timeout computations.

vss: Clean up timeout computations.

By passing the sched struct to vss_compute_timeout() we can get
rid of the static the_timeout struct in vss_compute_timeout(). The
previous patch, which made the timeout helper functions of sched.c
return whether the given barrier is in the past, allows to simplify
the timeout code of vss.c a bit.

The patch also combines the general timeout computations and the
computation for FEC slices into a single function.