arena: use the entire imported span

The source arena will round up to its quantum.  However, we would only
know that we got 'import_size', but we might get far more than that from
the parent.  If we round-up in advance, we'll know how much we are
getting.

Signed-off-by: Barret Rhoden <brho@cs.berkeley.edu>

kern/src/arena.c

diff --git a/kern/src/arena.c b/kern/src/arena.c
index 2c26ff3..267871f 100644
--- a/kern/src/arena.c
+++ b/kern/src/arena.c
@@ -687,9 +687,14 @@
 	void *span;
 	size_t import_size;
 
-	/* MAX check, in case size << scale overflows */
-	import_size = MAX(size, size << arena->import_scale);
 	if (arena->source) {
+		/* MAX check, in case size << scale overflows */
+		import_size = MAX(size, size << arena->import_scale);
+		/* The source will roundup to the nearest quantum.  We might as
+		 * well do it now so that we know about the extra space.
+		 * Otherwise we'd just waste the excess. */
+		import_size = MAX(import_size,
+				  ROUNDUP(import_size, arena->source->quantum));
 		span = arena->afunc(arena->source, import_size, flags);
 		if (!span)
 			return FALSE;