nfp: don't reuse pointers in ring dumping

We were reusing skb pointer when reading page frag, since ring
entries contain a union of a skb and frag pointer.  This can
be confusing to people reading the code.  Refactor the code
to read frag pointer directly.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>