skbuff: align sk_buff::cb to 64 bit and close some potential holes

The alignment requirement for 64-bit load/store instructions on ARM is
implementation defined. Some CPUs (such as Marvell Feroceon) do not
generate an exception, if such an instruction is executed with an
address that is not 64 bit aligned. In such a case, the Feroceon
corrupts adjacent memory, which showed up in my tests as a crash in the
rx path of ath9k that only occured with CONFIG_XFRM set.

This crash happened, because the first field of the mac80211 rx status
info in the cb is an u64, and changing it corrupted the skb->sp field.

This patch also closes some potential pre-existing holes in the sk_buff
struct surrounding the cb[] area.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -315,22 +315,23 @@ struct sk_buff {
 	struct sk_buff		*next;
 	struct sk_buff		*prev;
 
-	struct sock		*sk;
 	ktime_t			tstamp;
+
+	struct sock		*sk;
 	struct net_device	*dev;
 
-	unsigned long		_skb_dst;
-#ifdef CONFIG_XFRM
-	struct	sec_path	*sp;
-#endif
 	/*
 	 * This is the control buffer. It is free to use for every
 	 * layer. Please put your private variables there. If you
 	 * want to keep them across layers you have to do a skb_clone()
 	 * first. This is owned by whoever has the skb queued ATM.
 	 */
-	char			cb[48];
+	char			cb[48] __aligned(8);
 
+	unsigned long		_skb_dst;
+#ifdef CONFIG_XFRM
+	struct	sec_path	*sp;
+#endif
 	unsigned int		len,
 				data_len;
 	__u16			mac_len,