VMCI: Use struct_size() in kmalloc()

Use struct_size() instead of open coding.

Suggested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20240105164001.2129796-1-harshit.m.mogalapalli@oracle.com


Signed-off-by: Kees Cook <keescook@chromium.org>