Use the skb_frag_size accessor function rather than directly reading field

Use the skb_frag_size accessor function rather than directly reading field

There have been changes in the skb_frag_t struct. In newer kernels the
size field no longer exists.  Rather than trying to determine the name
of the field the __skb_frag_size tapset function now uses the
skb_frag_size accessor function instead.  This function has been in
kernels since 2011 and has been in RHEL kernels since RHEL6.