Fix secondary node selection for existing N+1

In case a secondary node is already N+1 failed, currently the node
selection will accept a node that cannot start (at all) the new instance
as valid. This is wrong, so we add a new simple check to prevent the
case of instance's memory size being higher than the node's free (not
available, which might be lower than 0 for N+1 failures) memory.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

Ganeti/HTools/Node.hs

@@ -374,6 +374,7 @@ addSec t inst pdx =
                                             T.dskWeight (Instance.util inst) }
     in case () of
          _ | new_dsk <= 0 || mDsk t > new_dp -> T.OpFail T.FailDisk
+           | Instance.mem inst >= old_mem -> T.OpFail T.FailMem
            | new_failn1 && not (failN1 t) -> T.OpFail T.FailMem
            | otherwise ->
                let new_slist = iname:sList t