net/af_xdp: forbid umem sharing for xsks with same context

net/af_xdp: forbid umem sharing for xsks with same context

AF_XDP PMDs who wish to share a UMEM must have a unique context
(ctx) ie. netdev,qid tuple. For instance, the following will not
work since both PMDs' contexts are identical.

  --vdev net_af_xdp0,iface=ens786f1,start_queue=0,shared_umem=1
  --vdev net_af_xdp1,iface=ens786f1,start_queue=0,shared_umem=1

Supporting this scenario would require locks, which would impact
the performance of the more typical cases - xsks with different
netdev,qid tuples.

Fixes: 74b46340e2d4 ("net/af_xdp: support shared UMEM")

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>