sampling: avoid needless modulu

Since Linux jiffies counters wrap at 32 bits, we take modulu 2 ** 32
when we calculate the difference between two consecustive samples.
Taking another modulu, after dividing by the time interval, is harmless
but also needless and confusing.

Change-Id: I31ec567836a4c9b59c113eff45e4baa653e399fa
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24193
Reviewed-by: Nir Soffer <nsoffer@redhat.com>