btrfs: use kvzalloc() to allocate clone_roots in btrfs_ioctl_send()

btrfs_ioctl_send() used open-coded kvzalloc implementation earlier.
The code was accidentally replaced with kzalloc() call [1]. Restore
the original code by using kvzalloc() to allocate sctx->clone_roots.

[1] https://patchwork.kernel.org/patch/9757891/#20529627

Fixes: 818e010b ("btrfs: replace opencoded kvzalloc with the helper")
CC: stable@vger.kernel.org # 4.14+
Signed-off-by: NDenis Efremov <efremov@linux.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/send.c

@@ -7149,7 +7149,7 @@ long btrfs_ioctl_send(struct file *mnt_file, struct btrfs_ioctl_send_args *arg)
 
 	alloc_size = sizeof(struct clone_root) * (arg->clone_sources_count + 1);
 
-	sctx->clone_roots = kzalloc(alloc_size, GFP_KERNEL);
+	sctx->clone_roots = kvzalloc(alloc_size, GFP_KERNEL);
 	if (!sctx->clone_roots) {
 		ret = -ENOMEM;
 		goto out;