iotests/046: Filter request length

For its concurrent requests, 046 has always filtered the offset,
probably because concurrent requests may settle in any order.  However,
it did not filter the request length, and so if requests with different
lengths settle in an unexpected order (notably the longer request before
the shorter request), the test fails (for no good reason).

Filter the length, too.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200918153323.108932-1-mreitz@redhat.com>