...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.

The entries should exist on any filesystem.

Fixes issue #62

We have to use the low-level function syscall.Getdents because
os.Readdir and friends will filter out "." and "..".

Futimes only takes microsecond resolution while the FUSE call
Utimens wants nanosecond precision. Switch from Futimes to futimens.

This is why UTIME_OMIT did not work - this change fixes the failure
of xfstests generic/221 (issue #60).

This function is neither provided by the syscall package nor x/sys/unix.

syscall.NsecToTimespec does not work properly for times
before 1970, see https://github.com/golang/go/issues/12777

This caused xfstests generic/258 to fail.

Fixes issue #58.

Merge into files.go

We can only do the lookup if we are sure that we will return the
result to the kernel, otherwise our lookup counts will go out of sync
with the kernel. As a bonus, this removes the allocation for the
EntryOut data structure, since we can write directly into the
readdirplus return data.

Add a port of the xfstest generic/257 that triggered this bug.

Fixes #57.

The old implementation was buggy and failed xfstests generic/001
(last 1kB of some files were filled with zeros).

The new one introduces a second splice layer to get rid of recursion
and passes generic/001.

Fixes issue #54.

Assuming that the loopback example binary is at /bin/loopbackfs,
you can mount like this:

	mount -t fuse.loopbackfs /tmp/mountpoint /tmp/original

However, "mount" passes an empty PATH to loopbackfs.
Trying the absolute path too fixes the issue.

This is in preparation of porting xfstests to go-fuse.

Now passes xfstests generic/053

Fixes issue #56

"df -T" output:

Before:

  Filesystem                 Type                   1K-blocks      Used  Available Use% Mounted on
  pathfs.pathInode           fuse.pathfs.pathInode    4015252   3219628     795624  81% /tmp/mountpoint

After:

  Filesystem                 Type             1K-blocks      Used  Available Use% Mounted on
  /tmp/original              fuse.loopbackfs    4015252   3219620     795632  81% /tmp/mountpoint

EINTR is commonly encountered when running under a debugger, so this
change makes it possible to do so without hitting spurious
EINTR errors.

Fixes #39.

This makes automated import rewriting more robust.

The code confuses the race-detector, and the work-arounds to fool the
race-detector fools humans. No conclusive benchmark data supports
the usefulness of the handle maps.