nfsd4: preserve __user annotation on cld downcall msg

cld_pipe_downcall(struct file *filp, const char __user *src, size_t mlen)

{

	struct cld_upcall *tmp, *cup;

	struct cld_msg *cmsg = (struct cld_msg *)src;

	struct cld_msg __user *cmsg = (struct cld_msg __user *)src;

	uint32_t xid;

	struct nfsd_net *nn = net_generic(filp->f_dentry->d_sb->s_fs_info,

						nfsd_net_id);