xsk: Move addrs from buffer pool to umem

Replicate the addrs pointer in the buffer pool to the umem. This mapping
will be the same for all buffer pools sharing the same umem. In the
buffer pool we leave the addrs pointer for performance reasons.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/1598603189-32145-8-git-send-email-magnus.karlsson@intel.com