Merge branch 'bpf-xsk-fixes'

Maciej Fijalkowski says:

====================
Cameron reported [0] that on fresh bpf-next he could not run multiple
xdpsock instances in Tx-only mode on single network interface with i40e
driver.

Turns out that Maxim's series [1] which was adding RCU protection around
ndo_xsk_wakeup added check against the __I40E_CONFIG_BUSY being set on
pf->state within i40e_xsk_wakeup() - if it's set, return -ENETDOWN.
Since this bit is set per PF when UMEM is being enabled/disabled, the
situation Cameron stumbled upon was that when he launched second xdpsock
instance, second UMEM was being registered, hence set __I40E_CONFIG_BUSY
which is now observed by first xdpsock and therefore xdpsock's kick_tx()
gets -ENETDOWN as errno.

-ENETDOWN currently is not allowed in kick_tx(), so we were exiting the
first application. Such exit means also XDP program being unloaded and
its dedicated resources, which caused an -ENXIO being return in the
second xdpsock instance.

Let's fix the issue from both sides - protect ourselves from future
xdpsock crashes by allowing for -ENETDOWN errno being set in kick_tx()
(patch 3) and from driver side, return -EAGAIN for the case where PF is
busy (patch 1).

Remove also doubled variable from xdpsock_user.c (patch 2).

Note that ixgbe seems not to be affected since UMEM registration sets
the busy/disable bit per ring, not per PF.

[0]: https://www.spinics.net/lists/xdp-newbies/msg01558.html
[1]: https://lore.kernel.org/netdev/20191217162023.16011-1-maximmi@mellanox.com/
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>