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>