drm/amdgpu: don't abuse current->group_leader

Cleanup and preparation to simplify the next changes.

- Use current->tgid instead of current->group_leader->pid

- Use get_task_pid(current, PIDTYPE_TGID) instead of
  get_task_pid(current->group_leader, PIDTYPE_PID)

Link: https://lkml.kernel.org/r/aXY_wKewzV5lCa5I@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Felix Kuehling <felix.kuehling@amd.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Christan König <christian.koenig@amd.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Steven Price <steven.price@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: oleg-nesterov

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

@@ -1421,7 +1421,7 @@ static int init_kfd_vm(struct amdgpu_vm *vm, void **process_info,
 			goto create_evict_fence_fail;
 		}
 
-		info->pid = get_task_pid(current->group_leader, PIDTYPE_PID);
+		info->pid = get_task_pid(current, PIDTYPE_TGID);
 		INIT_DELAYED_WORK(&info->restore_userptr_work,
 				  amdgpu_amdkfd_restore_userptr_worker);
 

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

@@ -2554,7 +2554,7 @@ void amdgpu_vm_set_task_info(struct amdgpu_vm *vm)
 	if (current->group_leader->mm != current->mm)
 		return;
 
-	vm->task_info->tgid = current->group_leader->pid;
+	vm->task_info->tgid = current->tgid;
 	get_task_comm(vm->task_info->process_name, current->group_leader);
 }