crush: pass parent r value for indep call

Pass down the parent's 'r' value so that we will sample different values in
the recursive call when the parent tries multiple times.  This avoids doing
useless work (calling multiple times and trying the same values).

Reflects ceph.git commit 2731d3030d7a3e80922b7f1b7756f9a4a124bac5.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>