exit: move wake_up_all() pidfd waiters into __unhash_process()

Move the pidfd notification out of __change_pid() and into
__unhash_process(). The only valid call to __change_pid() with a NULL
argument and PIDTYPE_PID is from __unhash_process(). This is a lot more
obvious than calling it from __change_pid().

Link: https://lore.kernel.org/20250411-work-pidfs-enoent-v2-1-60b2d3bb545f@kernel.org


Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>