net: enetc: fix buffer leaks with XDP_TX enqueue rejections

If the TX ring is congested, enetc_xdp_tx() returns false for the
current XDP frame (represented as an array of software BDs).

This array of software TX BDs is constructed in enetc_rx_swbd_to_xdp_tx_swbd
from software BDs freshly cleaned from the RX ring. The issue is that we
scrub the RX software BDs too soon, more precisely before we know that
we can enqueue the TX BDs successfully into the TX ring.

If we can't enqueue them (and enetc_xdp_tx returns false), we call
enetc_xdp_drop which attempts to recycle the buffers held by the RX
software BDs. But because we scrubbed those RX BDs already, two things
happen:

(a) we leak their memory
(b) we populate the RX software BD ring with an all-zero rx_swbd
    structure, which makes the buffer refill path allocate more memory.

enetc_refill_rx_ring
-> if (unlikely(!rx_swbd->page))
   -> enetc_new_page

That is a recipe for fast OOM.

Fixes: 7ed2bc80 ("net: enetc: add support for XDP_TX")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>