Use the overflow-protected struct_size() helper macro to compute the
allocation size of the vop2 data structure.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Sandy Huang <hjc@rock-chips.com>
Cc: Heiko Stübner <heiko@sntech.de>
Cc: David Airlie <airlied@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230223013533.1707706-1-jacob.e.keller@intel.com