rados: obey op_size for 'get'

Otherwise we try to read the whole object in one go, which doesn't bode
well for large objects (either non-optimal or simply broken).

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
(cherry picked from commit 234becd3447a679a919af458440bc31c8bd6b84f)
rados: remove unused "check_stdio" parameter

Signed-off-by: Dan Mick <dan.mick@inktank.com>
(cherry picked from commit bb860e49a7faeaf552538a9492ef0ba738c99760)