Add //pkg/sentry/fsimpl/overlay.

Major differences from existing overlay filesystems:

- Linux allows lower layers in an overlay to require revalidation, but not the
  upper layer. VFS1 allows the upper layer in an overlay to require
  revalidation, but not the lower layer. VFS2 does not allow any layers to
  require revalidation. (Now that vfs.MkdirOptions.ForSyntheticMountpoint
  exists, no uses of overlay in VFS1 are believed to require upper layer
  revalidation; in particular, the requirement that the upper layer support the
  creation of "trusted." extended attributes for whiteouts effectively required
  the upper filesystem to be tmpfs in most cases.)

- Like VFS1, but unlike Linux, VFS2 overlay does not attempt to make mutations
  of the upper layer atomic using a working directory and features like
  RENAME_WHITEOUT. (This may change in the future, since not having a working
  directory makes error recovery for some operations, e.g. rmdir, particularly
  painful.)

- Like Linux, but unlike VFS1, VFS2 represents whiteouts using character
  devices with rdev == 0; the equivalent of the whiteout attribute on
  directories is xattr trusted.overlay.opaque = "y"; and there is no equivalent
  to the whiteout attribute on non-directories since non-directories are never
  merged with lower layers.

- Device and inode numbers work as follows:

    - In Linux, modulo the xino feature and a special case for when all layers
      are the same filesystem:

        - Directories use the overlay filesystem's device number and an
          ephemeral inode number assigned by the overlay.

        - Non-directories that have been copied up use the device and inode
          number assigned by the upper filesystem.

        - Non-directories that have not been copied up use a per-(overlay,
          layer)-pair device number and the inode number assigned by the lower
          filesystem.

    - In VFS1, device and inode numbers always come from the lower layer unless
      "whited out"; this has the adverse effect of requiring interaction with
      the lower filesystem even for non-directory files that exist on the upper
      layer.

    - In VFS2, device and inode numbers are assigned as in Linux, except that
      xino and the samefs special case are not supported.

- Like Linux, but unlike VFS1, VFS2 does not attempt to maintain memory mapping
  coherence across copy-up. (This may have to change in the future, as users
  may be dependent on this property.)

- Like Linux, but unlike VFS1, VFS2 uses the overlayfs mounter's credentials
  when interacting with the overlay's layers, rather than the caller's.

- Like Linux, but unlike VFS1, VFS2 permits multiple lower layers in an
  overlay.

- Like Linux, but unlike VFS1, VFS2's overlay filesystem is
  application-mountable.

Updates #1199

PiperOrigin-RevId: 316019067

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