btrfs: cleanup, use kmalloc_array/kcalloc array helpers

Convert kmalloc(nr * size, ..) to kmalloc_array that does additional
overflow checks, the zeroing variant is kcalloc.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/raid56.c b/fs/btrfs/raid56.c
index 5264858..b4634c3 100644
--- a/fs/btrfs/raid56.c
+++ b/fs/btrfs/raid56.c
@@ -1807,8 +1807,7 @@
 	int err;
 	int i;
 
-	pointers = kzalloc(rbio->real_stripes * sizeof(void *),
-			   GFP_NOFS);
+	pointers = kcalloc(rbio->real_stripes, sizeof(void *), GFP_NOFS);
 	if (!pointers) {
 		err = -ENOMEM;
 		goto cleanup_io;