vfs: add vfs_ref_from_vp and use it in statfs

[mjg]

Summary

This will be 2 commits. I'm not fond of the name (vfs_ref_from_vp) but I don't see anything substantially better.

statfs keeps being used by 12.x world on each readdir call and this avoidably serializes with a lock against other threads (most notaby when they modify the content). With the change this is elided.

[mjg]

• also convert fstatfs