md: Fix missing unused status line of /proc/mdstat

Reading /proc/mdstat with a read buffer size that would not
fit the unused status line in the first read will skip this
line from the output.

So 'dd if=/proc/mdstat bs=64 2>/dev/null' will not print something
like: unused devices: <none>

Don't return NULL immediately in start() for v=2 but call
show() once to print the status line also for multiple reads.

Cc: stable@vger.kernel.org
Fixes: 1f4aace6 ("fs/seq_file.c: simplify seq_file iteration code and interface")
Signed-off-by: Jan Glauber <jglauber@digitalocean.com>
Signed-off-by: Song Liu <songliubraving@fb.com>

drivers/md/md.c

@@ -8153,7 +8153,11 @@ static void *md_seq_start(struct seq_file *seq, loff_t *pos)
 	loff_t l = *pos;
 	struct mddev *mddev;
 
-	if (l >= 0x10000)
+	if (l == 0x10000) {
+		++*pos;
+		return (void *)2;
+	}
+	if (l > 0x10000)
 		return NULL;
 	if (!l--)
 		/* header */