Fix typo when trying to create mount point directory to detect size

When trying an nfsroot installation this message is shown at the final
installation summary screen and the installation cannot be triggered:

Error: Cannot check free space in basic directory / (device
d46:/abuild/nfsroot_2), cannot start installation.

This patch was tested to fix this issue.

Signed-off-by: Thomas Renninger <trenn@suse.de>