DBObjectMap::clear_keys_header: use generate_new_header, not _generate_new_header

We aren't holding the header_lock here, so we need the locked version.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1077 from ceph/wip-7141

DBObjectMap::clear_keys_header: use generate_new_header, not _generate_n...

Reviewed-by: Sage Weil <sage@inktank.com>