bcache: switch from pages to folios in read_super()

bcache: switch from pages to folios in read_super()

Retrieve a folio from the page cache instead of a page. Removes a hidden
call to compound_head(). Then be sure to call folio_put() instead of
put_page() to release it. That doesn't save any calls to
compound_head(), just moves them around.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Coly Li <colyli@kernel.org>
Acked-back: Coly Li <colyli@kernel.org>
Link: https://lore.kernel.org/r/20250702024848.343370-1-colyli@kernel.org
[axboe: commit message massaging]
Signed-off-by: Jens Axboe <axboe@kernel.dk>