rgw: call processor->handle_data() again if needed

Fixes: #8937

Following the fix to #8928 we end up accumulating pending data that
needs to be written. Beforehand it was working fine because we were
feeding it with the exact amount of bytes we were writing.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
(cherry picked from commit 0553890e79b43414cc0ef97ceb694c1cb5f06bbb)