block: fix duplicate headers for /proc/partitions

seqf can be started multiple times for a read and the header should be
printed only for the initial one.  Fix it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/genhd.c

@@ -697,7 +697,7 @@ static void *show_partition_start(struct seq_file *seqf, loff_t *pos)
 	static void *p;
 
 	p = disk_seqf_start(seqf, pos);
-	if (!IS_ERR(p) && p)
+	if (!IS_ERR(p) && p && !*pos)
 		seq_puts(seqf, "major minor  #blocks  name\n\n");
 	return p;
 }