SCTP: Fix sctp_getsockopt_get_peer_addrs

This is the split out of the patch that we agreed I should split
out from my last patch.  It changes space_left to be computed in the same
way the to variable is.  I know we talked about changing space_left to an
int, but I think size_t is more appropriate, since we should never have
negative space in our buffer, and computing using offsetof means space_left
should now never drop below zero.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>

net/sctp/socket.c

@@ -4019,8 +4019,7 @@ static int sctp_getsockopt_peer_addrs(struct sock *sk, int len,
 		return -EINVAL;
 
 	to = optval + offsetof(struct sctp_getaddrs,addrs);
-	space_left = len - sizeof(struct sctp_getaddrs) -
-			offsetof(struct sctp_getaddrs,addrs);
+	space_left = len - offsetof(struct sctp_getaddrs,addrs);
 
 	list_for_each(pos, &asoc->peer.transport_addr_list) {
 		from = list_entry(pos, struct sctp_transport, transports);
@@ -4327,8 +4326,8 @@ static int sctp_getsockopt_local_addrs(struct sock *sk, int len,
 	}
 
 	to = optval + offsetof(struct sctp_getaddrs,addrs);
-	space_left = len - sizeof(struct sctp_getaddrs) -
-			 offsetof(struct sctp_getaddrs,addrs);
+	space_left = len - offsetof(struct sctp_getaddrs,addrs);
+
 	addrs = kmalloc(space_left, GFP_KERNEL);
 	if (!addrs)
 		return -ENOMEM;