qa: avoid a non-standard shell construct in rbd/iscsi_client.t

qa: avoid a non-standard shell construct in rbd/iscsi_client.t

dash which is used as /bin/sh on Ubuntu interprets "2&> /dev/null" as
an instruction to launch iscsiadm in the background.  While that is
mostly compensated by the following sleep, stderr isn't redirected to
/dev/null either -- the output gets polluted and the test fails.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>