Fix VM_BUG_ON in swap allocation.

The VM_BUG_ON was being triggered because our invocation of
inc_cluster_info_page for each page in a range of pages being allocated
was including the first page. The first page has already had
inc_cluster_info_page via scan_swap_map.

Signed-off-by: Nigel Cunningham <nigel@tuxonice.net>