gpu: nvgpu: Implement abstraction for finding TGID

Implement abstraction for finding the process ID of thread currently
being run. This is tracked for context switch tracing.

In Linux kernel this is implemented by returning TGID.

Change-Id: Ia6bcbd92c8cc25467694a35476e5d5f717194105
Signed-off-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1615985
Reviewed-by: svc-mobile-coverity <svc-mobile-coverity@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/common/linux/os_sched.c

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2017, NVIDIA CORPORATION.  All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ */
+
+#include <nvgpu/os_sched.h>
+
+#include <linux/sched.h>
+
+int nvgpu_current_pid(struct gk20a *g)
+{
+	return current->tgid;
+}