cache: ensure that the number of bucket locks does not exceed hash size

The size can be changed by side effect of modifying kern.maxvnodes.

Since numbucketlocks was not modified, setting a sufficiently low value
would give more locks than actual buckets, which would then lead to
corruption.

Force the number of buckets to be not smaller.

Note this should not matter for real world cases.

Reported and tested by: pho