crush: decode and initialize chooseleaf_vary_r

Commit e2b149cc

 ("crush: add chooseleaf_vary_r tunable") added the
crush_map::chooseleaf_vary_r field but missed the decode part.  This
lead to misdirected requests caused by incorrect raw crush mapping
sets.

Fixes: http://tracker.ceph.com/issues/8226

Reported-and-Tested-by: Dmitry Smirnov <onlyjob@member.fsf.org>
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>