selinux: use GFP_ATOMIC under spin_lock

The call tree here is:

sk_clone_lock()              <- takes bh_lock_sock(newsk);
xfrm_sk_clone_policy()
__xfrm_sk_clone_policy()
clone_policy()               <- uses GFP_ATOMIC for allocations
security_xfrm_policy_clone()
security_ops->xfrm_policy_clone_security()
selinux_xfrm_policy_clone()

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable@kernel.org
Signed-off-by: James Morris <james.l.morris@oracle.com>