media: venus: Use struct_size_t() helper in pkt_session_unset_buffers()

Prefer struct_size_t() over struct_size() when no pointer instance
of the structure type is present.

Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Reviewed-by: Vikash Garodia <quic_vgarodia@quicinc.com>
Link: https://lore.kernel.org/r/ZKBfoqSl61jfpO2r@work
Signed-off-by: Kees Cook <keescook@chromium.org>