NFSD: Add helper to decode NFSv4 verifiers

This helper will be used to simplify decoders in subsequent
patches.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4xdr.c

@@ -284,6 +284,18 @@ nfsd4_decode_nfstime4(struct nfsd4_compoundargs *argp, struct timespec64 *tv)
 	return nfs_ok;
 }
 
+static __be32
+nfsd4_decode_verifier4(struct nfsd4_compoundargs *argp, nfs4_verifier *verf)
+{
+	__be32 *p;
+
+	p = xdr_inline_decode(argp->xdr, NFS4_VERIFIER_SIZE);
+	if (!p)
+		return nfserr_bad_xdr;
+	memcpy(verf->data, p, sizeof(verf->data));
+	return nfs_ok;
+}
+
 static __be32
 nfsd4_decode_bitmap(struct nfsd4_compoundargs *argp, u32 *bmval)
 {
@@ -1047,14 +1059,16 @@ nfsd4_decode_open(struct nfsd4_compoundargs *argp, struct nfsd4_open *open)
 				goto out;
 			break;
 		case NFS4_CREATE_EXCLUSIVE:
-			READ_BUF(NFS4_VERIFIER_SIZE);
-			COPYMEM(open->op_verf.data, NFS4_VERIFIER_SIZE);
+			status = nfsd4_decode_verifier4(argp, &open->op_verf);
+			if (status)
+				return status;
 			break;
 		case NFS4_CREATE_EXCLUSIVE4_1:
 			if (argp->minorversion < 1)
 				goto xdr_error;
-			READ_BUF(NFS4_VERIFIER_SIZE);
-			COPYMEM(open->op_verf.data, NFS4_VERIFIER_SIZE);
+			status = nfsd4_decode_verifier4(argp, &open->op_verf);
+			if (status)
+				return status;
 			status = nfsd4_decode_fattr4(argp, open->op_bmval,
 						     ARRAY_SIZE(open->op_bmval),
 						     &open->op_iattr, &open->op_acl,