ovl: support idmapped layers

Now that overlay is able to take a layers idmapping into account allow
overlay mounts to be created on top of idmapped mounts.

Cc: <linux-unionfs@vger.kernel.org>
Tested-by: Giuseppe Scrivano <gscrivan@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
/* v2 */
- Turn on support for idmapped mounts in ovl_upper_idmap() helper here
  after we've introduced it earlier in the series and made it return the
  initial idmapping.

/* v3 */
2 files changed