Merge branch 'bpf-xdp-Report-bpf_prog-ID-in-IFLA_XDP'

Martin KaFai Lau says:

====================
bpf: xdp: Report bpf_prog ID in IFLA_XDP

This is the first usage of the new bpf_prog ID.  It is for
reporting the ID of a xdp_prog through netlink.

It rides on the existing IFLA_XDP.  This patch adds IFLA_XDP_PROG_ID
for the bpf_prog ID reporting.

It starts with changing the generic_xdp first.  After that,
the hardware driver is changed one by one.  Jakub Kicinski mentioned
that he will soon introduce XDP_ATTACHED_HW (on top of the existing
XDP_ATTACHED_DRV and XDP_ATTACHED_SKB)
and he is going to reuse the prog_attached for this purpose.
Hence, this patch set keeps the prog_attached even though
!!prog_id also implies there is xdp_prog attached.

I have tested with generic_xdp, mlx4 and mlx5.

v3:
1. Replace 'if' by '?' when checking the xdp_prog pointer
   as suggested by Jakub Kicinski (thanks!)

v2:
1. Remove READ_ONCE since it is alredy under rtnl lock
2. Keep prog_attached in 'struct netdev_xdp' as
   requested by Jakub Kicinski.  The existing prog_attached
   and the new prog_id are put under a struct for XDP_QUERY_PROG.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>