ibmvnic: Use 'skb_frag_address()' instead of hand coding it

'page_address(skb_frag_page()) + skb_frag_off()' can be replaced by an
equivalent 'skb_frag_address()' which is less verbose.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>