pktgen: refactor pg_init() code

This also shrinks the object size a little.

   text	   data	    bss	    dec	    hex	filename
  28834	    186	      8	  29028	   7164	net/core/pktgen.o
  28816	    186	      8	  29010	   7152	net/core/pktgen.o.AFTER

Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: "David Miller" <davem@davemloft.net>,
Signed-off-by: David S. Miller <davem@davemloft.net>