librgw: temporary fix RGWWrite

librgw: temporary fix RGWWrite

The current behavior of put_data_and_throttle leads to data
curruption, because the supplied buffer is apparently still being
written when the call returns--regardless of the value of
need_to_wait.

For now, do a buffer::copy of the supplied buffer, rather than the
original buffer::static, since that will at least be released
when no longer needed.

Probably this improves apparent rgw_write performance, but is not
what was originally intended.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>