rgw: clear bufferlist if need to go again

Fixes: #9201
Backport: firefly

We sometimes need to call RGWPutObjProcessor::handle_data() again,
so that we send the pending data. However, we failed to clear the buffer
that was already sent, thus it was resent. This triggers when using non
default pool alignments.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>