btrfs: Replace kmalloc with kmalloc_array

Replace kmalloc(size * nr, ) with kmalloc_array(nr, size), thus making
it easier to check is that the calculation doesn't wrap or return a smaller allocation

Signed-off-by: Dulshani Gunawardhana <dulshani.gunawardhana89@gmail.com>
Reviewed-by: Zach Brown <zab@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 78d2048..3712ef8 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1198,7 +1198,7 @@
 		ra = &file->f_ra;
 	}
 
-	pages = kmalloc(sizeof(struct page *) * max_cluster,
+	pages = kmalloc_array(max_cluster, sizeof(struct page *),
 			GFP_NOFS);
 	if (!pages) {
 		ret = -ENOMEM;