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>