mm: swap: Track allocation order for clusters

Add an `order` field to `struct swap_cluster_info`, which applies to
allocated clusters (i.e. those not on the free list) and tracks the swap
entry order that the cluster should be used to allocate. A future commit
will use this information to scan partially filled clusters to find
appropriate free swap entries for allocation. Note that it is still
possible that order-0 swap entries will be allocated in clusters that
indicate a higher order due to the order-0 scanning mechanism.

The maximum order we ever expect to see is 13 - PMD-size on arm64 with
64K base pages. 13 fits into 4 bits, so let's steal 4 unused flags bits
for this purpose to avoid making `struct swap_cluster_info` any bigger.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>