ovl: handle idmappings in ovl_xattr_{g,s}et()

When retrieving xattrs from the upper or lower layers take the relevant
mount's idmapping into account. We rely on the previously introduced
ovl_i_path_real() helper to retrieve the relevant path. This is needed
to support idmapped base layers with overlay.

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>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>