mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()

Now that we no longer have a convenient flag in the cluster to determine
if a folio is large, free_swap_and_cache() will take a reference and
lock a large folio much more often, which could lead to contention and
(e.g.) failure to split large folios, etc.

Let's solve that problem by batch freeing swap and cache with a new
function, free_swap_and_cache_nr(), to free a contiguous range of swap
entries together. This allows us to first drop a reference to each swap
slot before we try to release the cache folio. This means we only try to
release the folio once, only taking the reference and lock once - much
better than the previous 512 times for the 2M THP case.

Contiguous swap entries are gathered in zap_pte_range() and
madvise_free_pte_range() in a similar way to how present ptes are
already gathered in zap_pte_range().

While we are at it, let's simplify by converting the return type of both
functions to void. The return value was used only by zap_pte_range() to
print a bad pte, and was ignored by everyone else, so the extra
reporting wasn't exactly guaranteed. We will still get the warning with
most of the information from get_swap_device(). With the batch version,
we wouldn't know which pte was bad anyway so could print the wrong one.

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