shmem: convert shmem_get_partial_folio() to use shmem_get_folio()

Get rid of an unnecessary folio->page->folio conversion.

Link: https://lkml.kernel.org/r/20220902194653.1739778-26-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>