vm_kern: in unback, init cursor while unlocked

[dougm]

Summary

In kmem_unback(), take one small bit of code out of the region where the write lock is held.

[kib]

Something (un)related: Can we somehow automate checking for the issue where iterators are invalidated due to the object unlock? For instance, we can add the object' lock generation for INVARIANTS kernels, remember the current generation in the iterator on init, and assert that the lock generation does not change on next access to the iterator.