pid: use checkpoint_restore_ns_capable() for set_tid

Use the newly introduced capability CAP_CHECKPOINT_RESTORE to allow
using clone3() with set_tid set.

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Nicolas Viennot <Nicolas.Viennot@twosigma.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20200719100418.2112740-3-areber@redhat.com


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