Modify mallctl output length when needed

This is the only reason why `oldlenp` was designed to be in the form
of a pointer.

doc/jemalloc.xml.in

@@ -3608,7 +3608,8 @@ MAPPED_LIBRARIES:
             <constant>NULL</constant>, and <parameter>newlen</parameter> is too
             large or too small.  Alternatively, <parameter>*oldlenp</parameter>
             is too large or too small; in this case as much data as possible
-            are read despite the error.</para></listitem>
+            are read despite the error, with the amount of data read being
+            recorded in <parameter>*oldlenp</parameter>.</para></listitem>
           </varlistentry>
           <varlistentry>
             <term><errorname>ENOENT</errorname></term>

src/ctl.c

@@ -1494,6 +1494,7 @@ ctl_mtx_assert_held(tsdn_t *tsdn) {
 			size_t	copylen = (sizeof(t) <= *oldlenp)	\
 			    ? sizeof(t) : *oldlenp;			\
 			memcpy(oldp, (void *)&(v), copylen);		\
+			*oldlenp = copylen;				\
 			ret = EINVAL;					\
 			goto label_return;				\
 		}							\