sctp: do not update transport pathmtu if SPP_PMTUD_ENABLE is not set

Currently, in sctp_packet_config(), sctp_transport_pmtu_check() is
called to update transport pathmtu with dst's mtu when dst's mtu
has been changed by non sctp stack like xfrm.

However, this should only happen when SPP_PMTUD_ENABLE is set, no
matter where dst's mtu changed. This patch is to fix by checking
SPP_PMTUD_ENABLE flag before calling sctp_transport_pmtu_check().

Thanks Jacek for reporting and looking into this issue.

Fixes: 69fec32 ('Revert "sctp: remove sctp_transport_pmtu_check"')
Reported-by: Jacek Szafraniec <jacek.szafraniec@nokia.com>
Tested-by: Jacek Szafraniec <jacek.szafraniec@nokia.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>

net/sctp/output.c

@@ -104,8 +104,8 @@ void sctp_packet_config(struct sctp_packet *packet, __u32 vtag,
 		if (asoc->param_flags & SPP_PMTUD_ENABLE)
 			sctp_assoc_sync_pmtu(asoc);
 	} else if (!sctp_transport_pl_enabled(tp) &&
-		   !sctp_transport_pmtu_check(tp)) {
-		if (asoc->param_flags & SPP_PMTUD_ENABLE)
+		   asoc->param_flags & SPP_PMTUD_ENABLE)
+		if (!sctp_transport_pmtu_check(tp))
 			sctp_assoc_sync_pmtu(asoc);
 	}