tools/rados/rados.cc: use write_full for sync_write for ec pools

Fixes: #7650
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1437 from ceph/wip-7650

tools/rados/rados.cc: use write_full for sync_write for ec pools

Reviewed-by: Sage Weil <sage@inktank.com>