iscsi-target: Fix memory corruption in iscsit_logout_post_handler_diffcid

This patch fixes a bug in iscsit_logout_post_handler_diffcid() where
a pointer used as storage for list_for_each_entry() was incorrectly
being used to determine if no matching entry had been found.

This patch changes iscsit_logout_post_handler_diffcid() to key off
bool conn_found to determine if the function needs to exit early.

Reported-by: Joern Engel <joern@logfs.org>
Cc: <stable@vger.kernel.org> # v3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>