Protect em(4) refill timeout with splnet.

Protect em(4) refill timeout with splnet.

From time to time "pkt->pkt_m == NULL" or "m != NULL" assertions
were hit in the em driver.  Stack trace shows that em refill timeout
was interrupted by em interrupt.  Doing em_rxfill() and em_rxeof()
simultaneously cannot be correct.  Protect softclock in em_rxrefill()
with splnet().

OK mglocker@