nlmsg: Fix buffer update for NestedAttr.

For NestedAttr.PutAttr, the buffer may start pointing to another address
incase the buffer doesn't has enough capacity.
Updated NestedAttr to point to the correct address.

PiperOrigin-RevId: 842321572

Author: parth-opensrc

pkg/sentry/socket/netlink/nlmsg/message.go

@@ -219,6 +219,8 @@ func (n *NestedAttr) PutAttr(atype uint16, v marshal.Marshallable) {
 		buf: *n,
 	}
 	m.PutAttr(atype, v)
+	// m.buf may have been reallocated in PutAttr, so update the NestedAttr.
+	*n = m.buf
 }
 
 // PutAttrString adds s to the provided NestedAttr, creating nested attributes.
@@ -227,6 +229,8 @@ func (n *NestedAttr) PutAttrString(atype uint16, s string) {
 		buf: *n,
 	}
 	m.PutAttrString(atype, s)
+	// m.buf may have been reallocated in PutAttrString, so update the NestedAttr.
+	*n = m.buf
 }
 
 // MessageSet contains a series of netlink messages.