crush: invalidate rmap on create (and thus decode)

If we have an existing CrushWrapper object and decode from a bufferlist, 
reset build_rmaps so that they get rebuilt.

Remove the build_rmaps() all in decode that was useless on a redecode
(because have_rmaps == true in that case and it did nothing).

Fixes: #6442 Backport: dumpling Signed-off-by: Sage Weil <sage@inktank.com>