ceph: Remove call to PagePrivate2()

Use the folio that we already have to call folio_test_private_2()
instead.  This is the last call to PagePrivate2(), so replace its
PAGEFLAG() definition with FOLIO_FLAG().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20241002040111.1023018-6-willy@infradead.org


Signed-off-by: Christian Brauner <brauner@kernel.org>