btrfs: add 32-bit compat ioctl for BTRFS_IOC_GET_SUBVOL_INFO

btrfs: add 32-bit compat ioctl for BTRFS_IOC_GET_SUBVOL_INFO

On 64-bit kernels with 32-bit userspace, struct btrfs_ioctl_timespec is
laid out as 16 bytes (8B sec + 4B nsec + 4B trailing padding) instead of
the 12 bytes a 32-bit userspace expects, because the surrounding struct
is not packed. As a result, struct btrfs_ioctl_get_subvol_info_args has
a different size and layout in 32-bit userspace than in the 64-bit
kernel, and BTRFS_IOC_GET_SUBVOL_INFO returns garbage to 32-bit callers.

Mirror what was done for BTRFS_IOC_SET_RECEIVED_SUBVOL: add a packed
btrfs_ioctl_get_subvol_info_args_32 with btrfs_ioctl_timespec_32 fields,
define BTRFS_IOC_GET_SUBVOL_INFO_32 with that struct as the size
argument, factor the existing handler into a shared _btrfs_ioctl_get_
subvol_info() helper, and add btrfs_ioctl_get_subvol_info_32() which
fills the kernel struct and translates field-by-field into the 32-bit
struct before copy_to_user().

Signed-off-by: Daan De Meyer <daan@amutable.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>