addrconf: always initialize sysctl table data

When sysctl performs restrict writes, it allows to write from
a middle position of a sysctl file, which requires us to initialize
the table data before calling proc_dostring() for the write case.

Fixes: 3d1bec99 ("ipv6: introduce secret_stable to ipv6_devconf")
Reported-by: NSasha Levin <sasha.levin@oracle.com>
Acked-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Tested-by: NSasha Levin <sasha.levin@oracle.com>
Signed-off-by: NCong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -5369,13 +5369,10 @@ static int addrconf_sysctl_stable_secret(struct ctl_table *ctl, int write,
 		goto out;
 	}
 
-	if (!write) {
-		err = snprintf(str, sizeof(str), "%pI6",
-			       &secret->secret);
-		if (err >= sizeof(str)) {
-			err = -EIO;
-			goto out;
-		}
+	err = snprintf(str, sizeof(str), "%pI6", &secret->secret);
+	if (err >= sizeof(str)) {
+		err = -EIO;
+		goto out;
 	}
 
 	err = proc_dostring(&lctl, write, buffer, lenp, ppos);