rgw: fix error handling for GET with ?torrent

rgw: fix error handling for GET with ?torrent

GET requests with ?torrent should never be treated as a normal GET
request. if rgw_torrent_flag wasn't true when the object was uploaded,
we'll detect the lack of torrent data when reading from omap

conversely, if rgw_torrent_flag was true during upload but disabled
afterwards, we can still return the torrent data

Fixes: http://tracker.ceph.com/issues/23506
Signed-off-by: Casey Bodley <cbodley@redhat.com>