Fix a compiler warnings with NVALGRIND

Fix a compiler warnings with NVALGRIND

Fix compiler warnings when compiling with NVALGRIND defined and the
latest Valgrind header files.  Recently the Valgrind client request
implementation has been modified in order to not trigger compiler
warnings when building with gcc 4.6.  A side effect of that change is
that Valgrind client request macros that return a value have to be
cast to void in order to avoid a compiler warning.

For more information, see also:
* Valgrind manual about VALGRIND_MAKE_MEM_DEFINED (http://valgrind.org/docs/manual/mc-manual.html).
* Valgrind trunk r11755 (http://article.gmane.org/gmane.comp.debugging.valgrind.devel/13489).

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>