NFSv4.2: Fix a memory stomp in decode_attr_security_label

We must not change the value of label->len if it is zero, since that
indicates we stored a label.

Fixes: b4487b9354 ("nfs: Fix getxattr kernel panic and memory overflow")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/nfs4xdr.c

@@ -4236,12 +4236,10 @@ static int decode_attr_security_label(struct xdr_stream *xdr, uint32_t *bitmap,
 			return -EIO;
 		bitmap[2] &= ~FATTR4_WORD2_SECURITY_LABEL;
 		if (len < NFS4_MAXLABELLEN) {
-			if (label) {
-				if (label->len) {
+			if (label && label->len) {
 				if (label->len < len)
 					return -ERANGE;
 				memcpy(label->label, p, len);
-				}
 				label->len = len;
 				label->pi = pi;
 				label->lfs = lfs;