block: convert !IS_ERR(p) && p to !IS_ERR_NOR_NULL(p)

Signed-off-by: Yang Zhang <kthreadd@gmail.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/block/genhd.c b/block/genhd.c
index 5fa2b44..79b9e32 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -735,7 +735,7 @@
 	static void *p;
 
 	p = disk_seqf_start(seqf, pos);
-	if (!IS_ERR(p) && p && !*pos)
+	if (!IS_ERR_OR_NULL(p) && !*pos)
 		seq_puts(seqf, "major minor  #blocks  name\n\n");
 	return p;
 }