netfilter: ebtables: fix compat entry padding

On arm64, ebt_entry_{match,watcher,target} structs are 40 bytes long
while on 32-bit arm these structs have a size of 36 bytes.

COMPAT_XT_ALIGN() macro cannot be used here to determine the necessary
padding for the CONFIG_COMPAT because it imposes an 8-byte boundary
alignment, condition that is not found in 32-bit ebtables application.

Signed-off-by: Alin Nastac <alin.nastac@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>