htools: run IAllocator input through checkData

As the IAllocator backend is using a different data path than the
others, it doesn't get the full functionality that loadExternalData
does. This results in the current situation where checkData is not run
on the input cluster state, which means the node memory properties are
not correctly set, leading to no Xmem being set correctly.

This patch fixes this bug by adding a manual call to checkData.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>