goldfish_pipe: use GFP_ATOMIC under spin lock

The function get_free_pipe_id_locked() is called from
goldfish_pipe_open() with a lock is held, so we should
use GFP_ATOMIC instead of GFP_KERNEL.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>