btrfs-progs-4.5 introduces device delete by devid
for this reason btrfs_ioctl_vol_args_v2's name was encapsulated
in a union
this patch is for setting btrfs_ioctl_vol_args_v2's name
using a C function in order to preserve compatibility
with all btrfs-progs versions
Signed-off-by: Julio Montes <imc.coder@gmail.com>
(cherry picked from commit a038cccf88)