namespace: make timens_on_fork() return nothing

timens_on_fork() always return 0, and maybe not
need to judge the return value in copy_namespaces().

So make timens_on_fork() return nothing and do not
judge its return val in copy_namespaces().

Signed-off-by: Hui Su <sh_def@163.com>
Link: https://lore.kernel.org/r/20201117161750.GA45121@rlk


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