gpu: nvgpu: hold ch ref when getting ch from fd

Fix a race condition in gk20a_get_channel_from_file() that returns a
channel pointer from an fd: take a reference to the channel before
putting the file ref back. Now the caller is responsible of releasing
the channel reference eventually.

Also document why dbg_session_channel_data has to hold a ref to the
channel file instead of just the channel: that might deadlock if the fds
were closed in "wrong" order.

Change-Id: I8e91b809f5f7b1cb0c1487bd955ad6d643727a53
Signed-off-by: Konsta Holtta <kholtta@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1549290
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

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

@@ -354,6 +354,8 @@ static int nvgpu_gpu_ioctl_inval_icache(
 	nvgpu_mutex_acquire(&g->dbg_sessions_lock);
 	err = g->ops.gr.inval_icache(g, ch);
 	nvgpu_mutex_release(&g->dbg_sessions_lock);
+
+	gk20a_channel_put(ch);
 	return err;
 }
 
@@ -393,6 +395,7 @@ static int nvgpu_gpu_ioctl_set_debug_mode(
 		err = -ENOSYS;
 	nvgpu_mutex_release(&g->dbg_sessions_lock);
 
+	gk20a_channel_put(ch);
 	return err;
 }