netfilter: nft_exthdr: Search chunks in SCTP packets only

Since user space does not generate a payload dependency, plain sctp
chunk matches cause searching in non-SCTP packets, too. Avoid this
potential mis-interpretation of packet data by checking pkt->tprot.

Fixes: 133dc203 ("netfilter: nft_exthdr: Support SCTP chunks")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>