mptcp: return 0 instead of 'err' var

When 'err' is 0, it looks clearer to return '0' instead of the variable
called 'err'.

The behaviour is then not modified, just a clearer code.

By doing this, we can also avoid false positive smatch warnings like
this one:

  net/mptcp/pm_netlink.c:1169 mptcp_pm_parse_pm_addr_attr() warn: missing error code? 'err'
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Suggested-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/pm_netlink.c

@@ -1190,7 +1190,7 @@ static int mptcp_pm_parse_pm_addr_attr(struct nlattr *tb[],
 
 	if (!tb[MPTCP_PM_ADDR_ATTR_FAMILY]) {
 		if (!require_family)
-			return err;
+			return 0;
 
 		NL_SET_ERR_MSG_ATTR(info->extack, attr,
 				    "missing family");
@@ -1224,7 +1224,7 @@ static int mptcp_pm_parse_pm_addr_attr(struct nlattr *tb[],
 	if (tb[MPTCP_PM_ADDR_ATTR_PORT])
 		addr->port = htons(nla_get_u16(tb[MPTCP_PM_ADDR_ATTR_PORT]));
 
-	return err;
+	return 0;
 }
 
 int mptcp_pm_parse_addr(struct nlattr *attr, struct genl_info *info,

net/mptcp/sockopt.c

@@ -740,7 +740,7 @@ static int mptcp_setsockopt_v4_set_tos(struct mptcp_sock *msk, int optname,
 	}
 	release_sock(sk);
 
-	return err;
+	return 0;
 }
 
 static int mptcp_setsockopt_v4(struct mptcp_sock *msk, int optname,