xsk: Introduce batched Tx descriptor interfaces

Introduce batched descriptor interfaces in the xsk core code for the
Tx path to be used in the driver to write a code path with higher
performance. This interface will be used by the i40e driver in the
next patch. Though other drivers would likely benefit from this new
interface too.

Note that batching is only implemented for the common case when
there is only one socket bound to the same device and queue id. When
this is not the case, we fall back to the old non-batched version of
the function.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/1605525167-14450-5-git-send-email-magnus.karlsson@gmail.com