rcutorture: Check initrd/init instead of initrd only

If the build fails, we can end up with an empty initrd directory which
prevents the build script from operating again. Better rely on the
resulting init executable instead.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>