MINOR: ebtree: implement eb32sc_lookup_ge_or_first()

MINOR: ebtree: implement eb32sc_lookup_ge_or_first()

In the scheduler we always have to loop back to the beginning after
we don't find the last entry, so let's implement this in a new lookup
function instead. The resulting code is slightly faster, mostly due
to the fact that there's much less inlined code in the fast path.