fork: Use RCU_INIT_POINTER() instead of rcu_access_pointer()

Use RCU_INIT_POINTER() instead of rcu_access_pointer() in
copy_sighand().

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
[christian.brauner@ubuntu.com: edit commit message]
Link: https://lore.kernel.org/r/20200127175821.10833-1-madhuparnabhowmik10@gmail.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>