rgw/http: finish_request() after logging errors

rgw/http: finish_request() after logging errors

the http manager's background thread is processing responses and logging
any errors. but its call to finish_request() -> complete_request() drops
its reference with req_data->put() before logging the error message in
req_data->error_buf. if put() dropped the last reference, we'd access
error_buf after it's freed

Fixes: https://tracker.ceph.com/issues/67522
Signed-off-by: Casey Bodley <cbodley@redhat.com>