libceph: rename ceph_osd_request::r_linger_osd to r_linger_osd_item

So that:

req->r_osd_item --> osd->o_requests list
req->r_linger_osd_item --> osd->o_linger_requests list

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>