net: sun4i-emac: fix memory leak on bad packet

Upon reception of a new frame, the emac driver checks for a number
of error conditions, and flag the packet as "bad" if any of these
are present. It then allocates a skb unconditionally, but only uses
it if the packet is "good". On the error path, the skb is just forgotten,
and the system leaks memory.

The piece of junk I have on my desk seems to encounter such error
frequently enough so that the box goes OOM after a couple of days,
which makes me grumpy.

Fix this by moving the allocation on the "good_packet" path (and
convert it to netdev_alloc_skb while we're at it).

Tested on a random Allwinner A20 board.

Cc: Stefan Roese <sr@denx.de>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: <stable@vger.kernel.org> # 3.11+
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>