btrfs-progs: ioctl: add 64bit compat for SEND

btrfs-progs: ioctl: add 64bit compat for SEND

The ioctl value of SEND will be different on 32bit userspace and 64bit
kernel due to different pointer type width, that unfortunatelly made it
into the structure definition.

To maintain backward compatibility, we must do it in the 64bit->32bit
way, because we don't have the kernel side workardound like
SET_RECEIVED_SUBVOL has.  Changing value of SEND would then break
existing users of the raw ioctl.

The compatibility structure and ioctl should not be used, exists for
documentation, and testing.

Signed-off-by: David Sterba <dsterba@suse.com>