crush/CrushWrapper: ensure crush_choose_arg_map.size == max_buckets

crush/CrushWrapper: ensure crush_choose_arg_map.size == max_buckets

The crush/builder.c crush_add_bucket method resizes the max_buckets array
but a power of 2 when it has to expand, but the code in CrushWrapper was
assuming that if the array grew the pos for the new bucket would be the
last position in the new array.  This led to a situation where the
crush_choose_arg_map args array size didn't match max_buckets, and
eventually caused a crash.

Fixes: http://tracker.ceph.com/issues/38664
Signed-off-by: Sage Weil <sage@redhat.com>