netfilter: handle NF_REPEAT from nf_conntrack_in()

NF_REPEAT is only needed from nf_conntrack_in() under a very specific
case required by the TCP protocol tracker, we can handle this case
without returning to the core hook path. Handling of NF_REPEAT from the
nf_reinject() is left untouched.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>