percpu: fix a comment about the chunks ordering

Since the commit 3e54097b ("percpu: manage chunks based on
contig_bits instead of free_bytes") chunks are sorted based on the
size of the biggest continuous free area instead of the total number
of free bytes. Update the corresponding comment to reflect this.
Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

mm/percpu.c

@@ -99,7 +99,10 @@
 
 #include "percpu-internal.h"
 
-/* the slots are sorted by free bytes left, 1-31 bytes share the same slot */
+/*
+ * The slots are sorted by the size of the biggest continuous free area.
+ * 1-31 bytes share the same slot.
+ */
 #define PCPU_SLOT_BASE_SHIFT		5
 /* chunks in slots below this are subject to being sidelined on failed alloc */
 #define PCPU_SLOT_FAIL_THRESHOLD	3