UPSTREAM: mm: pass a folio to swap_readpage_bdev_async()

Make it plain that this takes the head page (which before this point
was just an assumption, but is now enforced by the compiler).

Link: https://lkml.kernel.org/r/20231213215842.671461-9-willy@infradead.org


Change-Id: I33060c99b0b563e04353048a58c0f0293f68fe24
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 3c3ebd82)
Bug: 313807618
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>