proc: Use PIDTYPE_TGID in next_tgid

Combine the pid_task and thes test has_group_leader_pid into a single
dereference by using pid_task(PIDTYPE_TGID).

This makes the code simpler and proof against needing to even think
about any shenanigans that de_thread might get up to.

Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>