target: Don't return an error if disabling unsupported features

If an attribute is present (but not yet supported) it should be OK
to write 0 (a no-op) to the attribute.

This is an issue because userspace should be able to save and restore all
set attribute values without error.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>