preload: don't try to free a zero-size region

in kmem_bootstrap_free() we round the start and end of the range to free
to avoid freeing unrelated records might share the first or last pages
of the range we are freeing.  this rounding can result in a range
that is zero or negative size (though negative becomes large positive
because the types are unsigned).  in this case there is nothing that
can actually be freed, so just return early.

Author: chucksilvers

sys/vm/vm_kern.c

@@ -948,6 +948,14 @@ kmem_bootstrap_free(vm_offset_t start, vm_size_t size)
 	end = trunc_page(start + size);
 	start = round_page(start);
 
+	/*
+	 * If rounding to page boundaries leaves us with nothing to free,
+	 * just return now.  vmem_add() will fail an assertion if we call it
+	 * with a zero size.
+	 */
+	if (end <= start)
+		return;
+
 #ifdef __amd64__
 	/*
 	 * Preloaded files do not have execute permissions by default on amd64.