NFSD: Add helper to decode OPEN's openflag4 argument

Refactor for clarity.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4xdr.c

@@ -988,6 +988,28 @@ nfsd4_decode_createhow4(struct nfsd4_compoundargs *argp, struct nfsd4_open *open
 	return nfs_ok;
 }
 
+static __be32
+nfsd4_decode_openflag4(struct nfsd4_compoundargs *argp, struct nfsd4_open *open)
+{
+	__be32 status;
+
+	if (xdr_stream_decode_u32(argp->xdr, &open->op_create) < 0)
+		return nfserr_bad_xdr;
+	switch (open->op_create) {
+	case NFS4_OPEN_NOCREATE:
+		break;
+	case NFS4_OPEN_CREATE:
+		status = nfsd4_decode_createhow4(argp, open);
+		if (status)
+			return status;
+		break;
+	default:
+		return nfserr_bad_xdr;
+	}
+
+	return nfs_ok;
+}
+
 static __be32 nfsd4_decode_share_access(struct nfsd4_compoundargs *argp, u32 *share_access, u32 *deleg_want, u32 *deleg_when)
 {
 	__be32 *p;
@@ -1082,19 +1104,9 @@ nfsd4_decode_open(struct nfsd4_compoundargs *argp, struct nfsd4_open *open)
 	status = nfsd4_decode_opaque(argp, &open->op_owner);
 	if (status)
 		goto xdr_error;
-	READ_BUF(4);
-	open->op_create = be32_to_cpup(p++);
-	switch (open->op_create) {
-	case NFS4_OPEN_NOCREATE:
-		break;
-	case NFS4_OPEN_CREATE:
-		status = nfsd4_decode_createhow4(argp, open);
-		if (status)
-			return status;
-		break;
-	default:
-		goto xdr_error;
-	}
+	status = nfsd4_decode_openflag4(argp, open);
+	if (status)
+		return status;
 
 	/* open_claim */
 	READ_BUF(4);