net: ptr_ring: otherwise safe empty checks can overrun array bounds

When running consumer and/or producer operations and empty checks in
parallel its possible to have the empty check run past the end of the
array. The scenario occurs when an empty check is run while
__ptr_ring_discard_one() is in progress. Specifically after the
consumer_head is incremented but before (consumer_head >= ring_size)
check is made and the consumer head is zeroe'd.

To resolve this, without having to rework how consumer/producer ops
work on the array, simply add an extra dummy slot to the end of the
array. Even if we did a rework to avoid the extra slot it looks
like the normal case checks would suffer some so best to just
allocate an extra pointer.

Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Fixes: c5ad119f ("net: sched: pfifo_fast use skb_array")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>