rcutorture/configinit: Fix build directory error message

[ Upstream commit 2adfa4210f8f35cdfb4e08318cc06b99752964c2 ]

The 'configinit.sh' script checks the format of optional argument for the
build directory, printing an error message if the format is not valid.
However, the error message uses the wrong variable, indicating an empty
string even though the user entered a non-empty (but erroneous) string.
This commit fixes the script to use the correct variable.

Fixes: c87b9c60 ("rcutorture: Add KVM-based test framework")

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>