connector/cn_proc: Use task_is_in_init_pid_ns()

This patch replaces open code with task_is_in_init_pid_ns() to check if
a task is in root PID namespace.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>