rework the filling of the rx ring. this switches us to having the cluster

allocation limited by per ifp statistics, ie, we're not guaranteed to have
mbufs in every slot on the rx ring.

instead of filling the ring with 256 mbufs all the time (about 512KB of
kva) when em is brought up, we give it 4. as demand grows we increase the
number of mbufs allowed on the ring. i will bet most users wont go above
50ish mbufs, so we're saving them 400KB of kva.

tested by many, including one on sparc64
ok claudio@ deraadt@ henning@ krw@

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