[NETFILTER]: nf_conntrack: fix helper structure alignment

Adding the alignment to the size doesn't make any sense, what it
should do is align the size of the conntrack structure to the
alignment requirements of the helper structure and return an
aligned pointer in nfct_help().

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>