xen/pvcalls: use GFP_ATOMIC under spin lock

A spin lock is taken here so we should use GFP_ATOMIC.

Fixes: 9774c6cc ("xen/pvcalls: implement accept command")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>