rgw: use s->content_length instead of s->length

Fixes: #7876
Need to use the actual content length, not the pointer to the string.
This was probably working because there's correlation to when
content_length > 0 to whether s->length is not null.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1544 from ceph/wip-7876

rgw: use s->content_length instead of s->length

Reviewed-by: Sage Weil <sage@inktank.com>