Fix for srun with --exclude and --nodes

Fix for srun allocating running within existing allocation with --exclude
option and --nnodes count small enough to remove more nodes.

    > salloc -N 8
    salloc: Granted job allocation 1000008
    > srun -N 2 -n 2 --exclude=tux3 hostname
    srun: error: Unable to create job step: Requested node configuration is not available

Patch from Phil Eckert, LLNL.