shmem: convert shmem_read_mapping_page_gfp() to use shmem_get_folio_gfp()

Saves a couple of calls to compound_head().

Link: https://lkml.kernel.org/r/20220902194653.1739778-24-willy@infradead.org


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