clone3: ensure copy_thread_tls is implemented

copy_thread implementations handle CLONE_SETTLS by reading the TLS
value from the registers containing the syscall arguments for
clone. This doesn't work with clone3 since the TLS value is passed
in clone_args instead.

Signed-off-by: Amanieu d'Antras <amanieu@gmail.com>
Cc: <stable@vger.kernel.org> # 5.3.x
Link: https://lore.kernel.org/r/20200102172413.654385-8-amanieu@gmail.com


Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>