mm/shmem: use a folio in shmem_unused_huge_shrink

When calling split_huge_page() we usually have to find the precise page,
but that's not necessary here because we only need to unlock and put the
folio afterwards.  Saves 231 bytes of text (20% of this function).

Link: https://lkml.kernel.org/r/20220504182857.4013401-17-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>