[NETFILTER]: fix ctnetlink 'create_expect' parsing

There was a stupid copy+paste mistake where we parse the MASK nfattr into
the "tuple" variable instead of the "mask" variable.  This patch fixes it.
Thanks to Pablo Neira.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>