i40e: Relax i40e_xsk_wakeup's return value when PF is busy

commit c77e9f09 upstream.

Return -EAGAIN instead of -ENETDOWN to provide a slightly milder
information to user space so that an application will know to retry the
syscall when __I40E_CONFIG_BUSY bit is set on pf->state.

Fixes: b3873a5b ("net/i40e: Fix concurrency issues between config flow and XSK")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/20200205045834.56795-2-maciej.fijalkowski@intel.com


[PG: note async_xmit ---> wakeup rename in mainline 9116e5e2.]
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>