misc: valgrind: fix valgrind.supp path

Signed-off-by: Sage Weil <sage@inktank.com>
misc: valgrind: fix cd behavior

The cd needs to happen at the beginning of the command, not at the end, or
else the funky wrapper scripts don't work right.

This also cleans up the command line construction a bit so that it is more
flexible, explicit, and hopefully less fragile.

Signed-off-by: Sage Weil <sage@inktank.com>