release_task: kill the no longer needed get/put_pid(thread_pid)

release_task: kill the no longer needed get/put_pid(thread_pid)

After the commit 7903f907a2260 ("pid: perform free_pid() calls outside
of tasklist_lock") __unhash_process() -> detach_pid() no longer calls
free_pid(), proc_flush_pid() can just use p->thread_pid without the
now pointless get_pid() + put_pid().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/20250411121857.GA10550@redhat.com
Reviewed-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>