Target/iser: Fix a wrong dereference in case discovery session is over iser

In case the discovery session is carried over iser, we can't
access the assumed network portal since the default portal is
used. In this case we don't really need to allocate the fastreg
pool, just prepare to the text pdu that will follow.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Reported-by: Alex Tabachnik <alext@mellanox.com>
Cc: stable@vger.kernel.org # 3.15+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>