sctp: subtract sctphdr len in sctp_transport_pl_hlen

sctp_transport_pl_hlen() is called to calculate the outer header length
for PL. However, as the Figure in rfc8899#section-4.4:

   Any additional
     headers         .--- MPS -----.
            |        |             |
            v        v             v
     +------------------------------+
     | IP | ** | PL | protocol data |
     +------------------------------+

                <----- PLPMTU ----->
     <---------- PMTU -------------->

Outer header are IP + Any additional headers, which doesn't include
Packetization Layer itself header, namely sctphdr, whereas sctphdr
is counted by __sctp_mtu_payload().

The incorrect calculation caused the link pathmtu to be set larger
than expected by t->pl.pmtu + sctp_transport_pl_hlen(). This patch
is to fix it by subtracting sctphdr len in sctp_transport_pl_hlen().

Fixes: d9e2e410 ("sctp: add the constants/variables and states and some APIs for transport")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sctp/sctp.h

@@ -626,7 +626,8 @@ static inline __u32 sctp_min_frag_point(struct sctp_sock *sp, __u16 datasize)
 
 static inline int sctp_transport_pl_hlen(struct sctp_transport *t)
 {
-	return __sctp_mtu_payload(sctp_sk(t->asoc->base.sk), t, 0, 0);
+	return __sctp_mtu_payload(sctp_sk(t->asoc->base.sk), t, 0, 0) -
+	       sizeof(struct sctphdr);
 }
 
 static inline void sctp_transport_pl_reset(struct sctp_transport *t)