fix skb fragment size access

The struct skb_frag_struct changed since Linux 5.4
commit 8842d285bafa9 ("net: Convert skb_frag_t to bio_vec") and
commit b8b576a16f79e ("net: Rename skb_frag_t size to bv_len")

The correct skb frag size accessors have been available since Linux 3.2
commit 9e903e085262 ("net: add skb frag size accessors")