buffer: use a folio in __find_get_block_slow()

Saves a call to compound_head() and may be needed to support block size >
PAGE_SIZE.

Link: https://lkml.kernel.org/r/20230612210141.730128-14-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Bob Peterson <rpeterso@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>