ceph: add a dedicated private data for netfs rreq

ceph: add a dedicated private data for netfs rreq

We need to save the 'f_ra.ra_pages' to expand the readahead window
later.

Cc: stable@vger.kernel.org
Fixes: 49870056005c ("ceph: convert ceph_readpages to ceph_readahead")
Link: https://lore.kernel.org/ceph-devel/20230504082510.247-1-sehuww@mail.scut.edu.cn
Link: https://www.spinics.net/lists/ceph-users/msg76183.html
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-and-tested-by: Hu Weiwen <sehuww@mail.scut.edu.cn>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>