LFS storage statistics are not exact

Description

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7754 adds new statistics for LFS objects on the project and group level. Each unique LFS file is only stored once and shared across all projects linking to it, but their size is currently still counted more than once against each related project/group.

Links / references

Relevant comments:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7754#note_19157888
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7754#note_19161964
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7754#note_19173251