netfs, ceph: Partially revert "netfs: Replace PG_fscache by setting folio->private and marking dirty"

This partially reverts commit 2ff1e975.

In addition to reverting the removal of PG_private_2 wrangling from the
buffered read code[1][2], the removal of the waits for PG_private_2 from
netfs_release_folio() and netfs_invalidate_folio() need reverting too.

It also adds a wait into ceph_evict_inode() to wait for netfs read and
copy-to-cache ops to complete.

Fixes: 2ff1e975 ("netfs: Replace PG_fscache by setting folio->private and marking dirty")
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/3575457.1722355300@warthog.procyon.org.uk [1]
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=8e5ced7804cb9184c4a23f8054551240562a8eda [2]
Link: https://lore.kernel.org/r/20240814203850.2240469-2-dhowells@redhat.com
cc: Max Kellermann <max.kellermann@ionos.com>
cc: Ilya Dryomov <idryomov@gmail.com>
cc: Xiubo Li <xiubli@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: ceph-devel@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Signed-off-by: Christian Brauner <brauner@kernel.org>