netfilter: ctnetlink: reject new conntrack request with different l4proto

Currently, user can add a conntrack with different l4proto via nfnetlink.
For example, original tuple is TCP while reply tuple is SCTP. This is
invalid combination, we should report EINVAL to userspace.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>