ceph: use attach/detach_page_private for tracking snap context

There is some ambiguity around the use of PagePrivate. It's
generally expected in core code that if PagePrivate is set then
you have a reference to it. It's not clear that ceph always
does (and I believe it may not).

Change ceph to use attach/detach_page_private so that we keep a
reference to the page until the snap context is detached.

Link: https://lore.kernel.org/ceph-devel/2503810.1616508988@warthog.procyon.org.uk/T/#mf29e5abbb0ec8035cde0de30778690de7d956f84


Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>