+static long btrfs_ioctl_set_received_subvol(struct file *file,
+					    void __user *arg)
+{
+	struct btrfs_ioctl_received_subvol_args *sa = NULL;

+	ret = copy_to_user(arg, sa, sizeof(*sa));

+struct btrfs_ioctl_received_subvol_args {
+	char	uuid[BTRFS_UUID_SIZE];	/* in */
+	__u64	stransid;		/* in */
+	__u64	rtransid;		/* out */
+	struct timespec stime;		/* in */
+	struct timespec rtime;		/* out */
+	__u64	reserved[16];
+};

Careful, timespec will be different sizes in 32bit userspace and a 64bit
kernel.  I'd use btrfs_timespec to get a fixed size timespec and avoid
all the compat_timespec noise.  (I'd then also worry about padding and
might pack the struct.. I always lose track of the best practice across
all archs.)

- z