netfilter: conntrack: avoid unneeded nf_conntrack_l4proto lookups

after removal of the packet and invert function pointers, several
places do not need to lookup the l4proto structure anymore.

Remove those lookups.
The function nf_ct_invert_tuplepr becomes redundant, replace
it with nf_ct_invert_tuple everywhere.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>