fixup! Revoke fully unmapped reservations

Better document the quarantine path in vm_map_remove_locked.

sys/vm/vm_map.c

@@ -4720,7 +4720,15 @@ vm_map_remove_locked(vm_map_t map, vm_offset_t start, vm_offset_t end)
 #ifdef CHERI_CAPREVOKE
 		if (quarantine_unmapped_reservations &&
 		    start < VM_MAXUSER_ADDRESS) {
+			/*
+			 * If we're here, [start, end) was the last mapped
+			 * range in reservation and it has transitioned to
+			 * fully unmapped with a new entry.  Look up the
+			 * entry and quarantine it.
+			 */
 			vm_map_lookup_entry(map, start, &entry);
+			KASSERT(entry->start <= start && entry->end >= end,
+			    ("entry doesn't cover removed range"));
 			vm_map_entry_quarantine(map, entry);
 		} else
 #endif