swap_state: convert free_swap_cache() to use a folio

Saves several calls to compound_head().

Link: https://lkml.kernel.org/r/20220902194653.1739778-40-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>