Add reference counting utility to VFS2.

The utility has several differences from the VFS1 equivalent:
- There are no weak references, which have a significant overhead
- In order to print useful debug messages with the type of the reference-
  counted object, we use a generic Refs object with the owner type as a
  template parameter. In vfs1, this was accomplished by storing a type name
  and caller stack directly in the ref count (as in vfs1), which increases the
  struct size by 6x. (Note that the caller stack was needed because fs types
  like Dirent were shared by all fs implementations; in vfs2, each impl has
  its own data structures, so this is no longer necessary.)

As an example, the utility is added to tmpfs.inode.

Updates #1486.

PiperOrigin-RevId: 324906582

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