target/qla2xxx: Make demo_mode_login_only modifiable

Changing this attribute to 0 will mean that all initiators that login
to the target while the target has generate_node_acls=1 will see all
TPG LUNs, which may be the desired behavior in some cases.

(nab: Apply patch without macro changes)

Reported-by: Craig Watson <craig.watson@vanguard-rugged.com>
Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>