libceph: fix pg_temp updates

Usually, we are adding pg_temp entries or removing them.  Occasionally they
update.  In that case, osdmap_apply_incremental() was failing because the
rbtree entry already exists.

Fix by removing the existing entry before inserting a new one.

Fixes http://tracker.newdream.net/issues/2446

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>