crushtool dumps core with non-unique bucket IDs

Return -EEXIST on duplicate ID
BUG FIX: crush_add_bucket() mixes error returns and IDs
  Add optional argument to return generated ID

fixes: #6246

Signed-off-by: David Zafman <david.zafman@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>