A volume that has single-use access mode is feasibility checked during
scheduling to ensure that only a single reader or writer claim
exists. However, because feasibility checking is done one alloc at a
time before the plan is written, a job that's misconfigured to have
count > 1 that mounts one of these volumes will pass feasibility
checking.
Enforce the check at validation time instead to prevent us from even
trying to evaluation a job that's misconfigured this way.
