rgw: disable prefetch in rgw_file

rgw: disable prefetch in rgw_file

Each call to rgw_read (rgw_file.cc) invokes three calls to RGWRados::get_obj_state with s->prefetch_data=true. It results in great read amplification. If length argument in rgw_read call is smaller than rgw_max_chunk_size, then the amplification is threefold.

Signed-off-by: Kajetan Janiak <kjaniak@cloudferro.com>
(cherry picked from commit f915e21e5a1baf6030c1407b3058d4f58c638df9)

Conflicts:
	src/rgw/rgw_op.cc

Cherry-pick notes:
- Octopus sets prfetch data flag through Rados method vs. method on object