rgw: error out if frontend did not send all data

rgw: error out if frontend did not send all data

Fixes: #11851
The civetweb mg_write() doesn't return error when it can't flush all data
to the user, it just sends the total number of bytes written. Modified the
client io to return total number of bytes and return an error if didn't
send anything.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
(cherry picked from commit daa679c3dd3770a6d5421e2cc9a36924f4550439)