sctp: allow unsetting sockopt MAXSEG

RFC 6458 Section 8.1.16 says that setting MAXSEG as 0 means that the user
is not limiting it, and not that it should set to the *current* maximum,
as we are doing.

This patch thus allow setting it as 0, effectively removing the user
limit.

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -3211,7 +3211,6 @@ static int sctp_setsockopt_mappedv4(struct sock *sk, char __user *optval, unsign
 static int sctp_setsockopt_maxseg(struct sock *sk, char __user *optval, unsigned int optlen)
 {
 	struct sctp_sock *sp = sctp_sk(sk);
-	struct sctp_af *af = sp->pf->af;
 	struct sctp_assoc_value params;
 	struct sctp_association *asoc;
 	int val;
@@ -3249,12 +3248,6 @@ static int sctp_setsockopt_maxseg(struct sock *sk, char __user *optval, unsigned
 	}
 
 	if (asoc) {
-		if (val == 0) {
-			val = asoc->pathmtu - af->net_header_len;
-			val -= af->ip_options_len(sk);
-			val -= sizeof(struct sctphdr) +
-			       sctp_datachk_len(&asoc->stream);
-		}
 		asoc->user_frag = val;
 		sctp_assoc_update_frag_point(asoc);
 	} else {