net/i40e: Fix concurrency issues between config flow and XSK

net/i40e: Fix concurrency issues between config flow and XSK

Use synchronize_rcu to wait until the XSK wakeup function finishes
before destroying the resources it uses:

1. i40e_down already calls synchronize_rcu. On i40e_down either
__I40E_VSI_DOWN or __I40E_CONFIG_BUSY is set. Check the latter in
i40e_xsk_wakeup (the former is already checked there).

2. After switching the XDP program, call synchronize_rcu to let
i40e_xsk_wakeup exit before the XDP program is freed.

3. Changing the number of channels brings the interface down (see
i40e_prep_for_reset and i40e_pf_quiesce_all_vsi).

4. Disabling UMEM sets __I40E_CONFIG_BUSY, too.

Signed-off-by: Maxim Mikityanskiy <[email protected]>
Signed-off-by: Björn Töpel <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]