btrfs.c: print file descriptors using printfd

btrfs.c: print file descriptors using printfd

* btrfs.c (btrfs_ioctl): Print struct btrfs_ioctl_send_args.send_fd,
struct btrfs_ioctl_vol_args.fd, and struct btrfs_ioctl_vol_args_v2.fd
using printfd.
* tests/btrfs.c (btrfs_print_vol_args_v2, btrfs_test_subvol_ioctls,
btrfs_test_device_ioctls, btrfs_test_clone_ioctls,
btrfs_test_send_ioctl): Update.