kernel - Fix resident_count problems with kernel_pmap & related crashes
* kernel_pmap.pm_stats.resident_count could get out of sync due to the
lwbuf code calling kmem_free() on KVM with pages that were entered
via pmap_kenter(). Delete the pages before calling kmem_free().
This in turn caused various failures and kernel panics due to conditionals
based on resident_count == 0. Heavy sendfile() activity could trigger
the bug but it was taking a few days for the resident_count to decrement
to zero.
* Fix a similar issue in zdestroy() on ZONE_INTERRUPT zones. Nothing calls
this routine so it was not a vector for crashes.
* Document the case.