amd/amdgpu_ctx: Use struct_size() helper and kmalloc() (v2)

Make use of the new struct_size() helper instead of the offsetof() idiom.
Also, use kmalloc() instead of kcalloc().

v2: squash in kzalloc fix

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c

@@ -100,7 +100,7 @@ static int amdgpu_ctx_init_entity(struct amdgpu_ctx *ctx, u32 hw_ip,
 	enum drm_sched_priority priority;
 	int r;
 
-	entity = kcalloc(1, offsetof(typeof(*entity), fences[amdgpu_sched_jobs]),
+	entity = kzalloc(struct_size(entity, fences, amdgpu_sched_jobs),
 			 GFP_KERNEL);
 	if (!entity)
 		return  -ENOMEM;