skb_array: array based FIFO for skbs

A simple array based FIFO of pointers.  Intended for net stack so uses
skbs for type safety. Implemented as a set of wrappers around ptr_ring.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>