xfs: hide private inodes from bulkstat and handle functions

We're about to start adding functionality that uses internal inodes that
are private to XFS.  What this means is that userspace should never be
able to access any information about these files, and should not be able
to open these files by handle.

To prevent users from ever finding the file or mis-interactions with the
security apparatus, set S_PRIVATE on the inode.  Don't allow bulkstat,
open-by-handle, or linking of S_PRIVATE files into the directory tree.
This should keep private inodes actually private.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>