rgw: bucket recreation should not clobber bucket info

Fixes: #4039
User's list of buckets is getting modified even if bucket already
exists. This fix removes the newly created directory object, and
makes sure that user info's data points at the correct bucket.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
rgw: a tool to fix clobbered bucket info in user's bucket list

This fixes bad entries in user's bucket list that may have occured
due to issue #4039. Syntax:

 $ radosgw-admin user check --uid=<uid> [--fix]

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>