igb: Always call igb_xdp_ring_update_tail() under Tx lock

Always call igb_xdp_ring_update_tail() under __netif_tx_lock, add a comment
and lockdep assert to indicate that. This is needed to share the same TX
ring between XDP, XSK and slow paths. Furthermore, the current XDP
implementation is racy on tail updates.

Fixes: 9cbc948b ("igb: add XDP support")
Signed-off-by: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
[Kurt: Add lockdep assert and fixes tag]
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: George Kuruvinakunnel <george.kuruvinakunnel@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>