ofproto: facet_execute() stats changes.

facet_execute() should rely on facet_update_stats() to update the
relevant facet's used parameter.

Before this patch, in very rare cases, the current practice could
lead to assertion failures.  The used value of a facet could be
slightly in the future due to the clocks being out of sync between
the kernel and user space.  Thus manually setting the used
parameter of a facet could put it slightly behind an already pushed
used value causing an assertion failure.

Bug #4745.