nfp: xsk: add an array of xsk buffer pools to each data path

Each data path needs an array of xsk pools to track if an xsk socket is
in use. Add this array and make sure it's handled correctly when the
data path is duplicated.

Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>