percpu: add block level scan_hint

Fragmentation can cause both blocks and chunks to have an early
first_firee bit available, but only able to satisfy allocations much
later on. This patch introduces a scan_hint to help mitigate some
unnecessary scanning.

The scan_hint remembers the largest area prior to the contig_hint. If
the contig_hint == scan_hint, then scan_hint_start > contig_hint_start.
This is necessary for scan_hint discovery when refreshing a block.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>