create_tarballs: fix nested submodules
The vtk-m submodule was not placed in the right location in the tarball
because $sm_path
is the path to the submodule from the parent
repository. Instead, use $displaypath
which is the path from the
current working directory (which should be the root of the repo).
Fixes: #20406 (closed)
Cc: @vbolea
Backport: release