netfilter: ctnetlink: fix ctnetlink_parse_tuple() warning

net/netfilter/nf_conntrack_netlink.c: In function 'ctnetlink_parse_tuple':
net/netfilter/nf_conntrack_netlink.c:832:11: warning: comparison between 'enum ctattr_tuple' and 'enum ctattr_type'

Use ctattr_type for the 'type' parameter since that's the type of all attributes
passed to this function.

Signed-off-by: Patrick McHardy <kaber@trash.net>