xdr_string: don't leak strings with xdr_free

Historically (and in a small amount of older software such as OpenAFS),
developers would attempt to free XDR strings with

	xdr_free((xdrproc_t)xdr_string, &string)

This resulted in xdr_free calling xdr_string with only two intentional
arguments and whatever was left in the third argument register.  If the
register held a sufficently small number, xdr_string would return FALSE
and not free the string (no one checks the return values).

Software should instead free strings with:

	xdr_free((xdrproc_t)xdr_wrapstring, &string)

Because buggy software exists in the wild, act as though xdr_wrapstring
was used in the XDR_FREE case and plug these leaks.

Reviewed by:	kib
MFC after:	3 days
Effort:		CHERI upstreaming
Sponsored by:	Innovate UK
Differential Revision:	https://reviews.freebsd.org/D54825

lib/libc/xdr/xdr.c

@@ -696,6 +696,13 @@ xdr_string(XDR *xdrs, char **cpp, u_int maxsize)
 		if (sp == NULL) {
 			return(TRUE);	/* already free */
 		}
+		/*
+		 * XXX: buggy software may call this without a third
+		 * argument via xdr_free().  Ignore maxsize since it may
+		 * be invalid.  Otherwise, if it's very small, we might
+		 * fail to free the string.
+		 */
+		maxsize = RPC_MAXDATASIZE;
 		/* FALLTHROUGH */
 	case XDR_ENCODE:
 		size = strlen(sp);

sys/xdr/xdr.c

@@ -620,6 +620,13 @@ xdr_string(XDR *xdrs, char **cpp, u_int maxsize)
 		if (sp == NULL) {
 			return(TRUE);	/* already free */
 		}
+		/*
+		 * XXX: buggy software may call this without a third
+		 * argument via xdr_free().  Ignore maxsize since it may
+		 * be invalid.  Otherwise, if it's very small, we might
+		 * fail to free the string.
+		 */
+		maxsize = RPC_MAXDATASIZE;
 		/* FALLTHROUGH */
 	case XDR_ENCODE:
 		size = strlen(sp);