...if it has already been set by the filesystem.

"out.Ino" is the user-facing inode number that is displayed by "ls -li".

This change allows loopback filesystems to expose the inode number of
the underlying filesystem.
As a "real" inode number, this one is stable for the lifetime of the file,
even across remounts.

libfuse has the `use_ino` option that allows the inode numbers to be set
arbitrarily as well. This has been used by EncFS for years.

The Linux kernel stores the inode number in `orig_ino`. Grepping in
`fs/fuse` shows that it is only read once in `fuse_update_attributes`,
and only if the `stat` argument is not NULL. There is only one caller
that passes that argument: `fuse_getattr`.

In short, the kernel does not use the inode number for anything but
reporting it via `stat()`, hence setting the inode number is safe.

This functionality will be used for improving log output in gocryptfs,
where file names cannot be used because they are encrypted.