selftests/fscontext_ns: expect EINVAL for overlay descendant userns create

Commit a518e63c3775 ("ovl: return EINVAL instead of EIO in case of
mismatched user_ns") changed the errno for completing an overlay
fscontext from another user namespace. Update the overlay test variant
to match binfmt_misc.

Reported-by: kernel test robot <yi1.lai@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202611020115.2281e158-lkp@intel.com
Fixes: a518e63c3775 ("ovl: return EINVAL instead of EIO in case of mismatched user_ns")
Cc: stable@vger.kernel.org
Signed-off-by: Amir Goldstein <amir73il@gmail.com>