idpf: fix corrupted frames and skb leaks in singleq mode

idpf_ring::skb serves only for keeping an incomplete frame between
several NAPI Rx polling cycles, as one cycle may end up before
processing the end of packet descriptor. The pointer is taken from
the ring onto the stack before entering the loop and gets written
there after the loop exits. When inside the loop, only the onstack
pointer is used.
For some reason, the logics is broken in the singleq mode, where the
pointer is taken from the ring each iteration. This means that if a
frame got fragmented into several descriptors, each fragment will have
its own skb, but only the last one will be passed up the stack
(containing garbage), leaving the rest leaked.
Then, on ifdown, rxq::skb is being freed only in the splitq mode, while
it can point to a valid skb in singleq as well. This can lead to a yet
another skb leak.
Just don't touch the ring skb field inside the polling loop, letting
the onstack skb pointer work as expected: build a new skb if it's the
first frame descriptor and attach a frag otherwise. On ifdown, free
rxq::skb unconditionally if the pointer is non-NULL.

Fixes: a5ab9ee0

 ("idpf: add singleq start_xmit and napi poll")
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Michal Kubiak <michal.kubiak@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Tested-by: Scott Register <scott.register@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>