Build fixes for skb_frag_size conversion

I missed a few places.  One is in some ifdeffed code which will probably
never be re-enabled; the others are in drivers which can't currently be
compiled on x86.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>