NFSv4.2: Fix a memory stomp in decode_attr_security_label

jira LE-1907
Rebuild_History Non-Buildable kernel-rt-5.14.0-284.30.1.rt14.315.el9_2
commit-author Trond Myklebust <trond.myklebust@hammerspace.com>
commit 43c1031

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

Fixes: b4487b9 ("nfs: Fix getxattr kernel panic and memory overflow")
	Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
(cherry picked from commit 43c1031)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: PlaidCat

fs/nfs/nfs4xdr.c

@@ -4219,12 +4219,10 @@ static int decode_attr_security_label(struct xdr_stream *xdr, uint32_t *bitmap,
 		if (unlikely(!p))
 			return -EIO;
 		if (len < NFS4_MAXLABELLEN) {
-			if (label) {
-				if (label->len) {
-					if (label->len < len)
-						return -ERANGE;
-					memcpy(label->label, p, 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;