tcm_loop: Fix wrong I_T nexus association

tcm_loop has the I_T nexus associated with the HBA. This causes
commands to become misdirected if the HBA has more than one
target portal group; any command is then being sent to the
first target portal group instead of the correct one.

The nexus needs to be associated with the target portal group
instead.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Cc: <stable@vger.kernel.org> # 3.0+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>