ceph: move ceph_osdc_{read,write}pages to ceph.ko

Since these helpers are only used by ceph.ko, move them there and
rename them with _sync_ qualifiers.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>