swapfile: convert try_to_unuse() to use a folio

Saves five calls to compound_head().

Link: https://lkml.kernel.org/r/20220902194653.1739778-35-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>