When converting ns,us,ms to sbt, return the ceil() of the result

When converting ns,us,ms to sbt, return the ceil() of the result
rather than the floor(). Returning the floor means that
sbttoX(Xtosbt(y)) != y for almost all values of y.  In practice, this
results in a difference of at most 1 in the lsb of the sbintime_t.
This difference is meaningless for all current users of these
functions, but is important for the newly introduced sysctl conversion
routines which implicitly rely on the transformation being idempotent.

Sponsored by: Netflix, Inc