mm: swap: Remove struct percpu_cluster

struct percpu_cluster stores the index of cpu's current cluster and the
offset of the next entry that will be allocated for the cpu. These two
pieces of information are redundant because the cluster index is just
(offset / SWAPFILE_CLUSTER). The only reason for explicitly keeping the
cluster index is because the structure used for it also has a flag to
indicate "no cluster". However this data structure also contains a spin
lock, which is never used in this context, as a side effect the code
copies the spinlock_t structure, which is questionable coding practice
in my view.

So let's clean this up and store only the next offset, and use a
sentinal value (SWAP_NEXT_NULL) to indicate "no cluster". SWAP_NEXT_NULL
is chosen to be 0, because 0 will never be seen legitimately; The first
page in the swap file is the swap header, which is always marked bad to
prevent it from being allocated as an entry. This also prevents the
cluster to which it belongs being marked free, so it will never appear
on the free list.

This change saves 16 bytes per cpu. And given we are shortly going to
extend this mechanism to be per-cpu-AND-per-order, we will end up saving
16 * 9 = 144 bytes per cpu, which adds up if you have 256 cpus in the
system.

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