netfilter: xtables: shorten up return clause

The return value of nf_ct_l3proto_get can directly be returned even in
the case of success.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>

diff --cc net/ipv4/netfilter/ipt_CLUSTERIP.c
Simple merge

diff --cc net/netfilter/xt_CONNSECMARK.c
Simple merge

diff --cc net/netfilter/xt_connbytes.c
Simple merge

diff --cc net/netfilter/xt_connmark.c
Simple merge

diff --cc net/netfilter/xt_conntrack.c
Simple merge

diff --cc net/netfilter/xt_state.c
Simple merge