[SCSI] iscsi cls: sysfs group is_visible callout for conn attrs

The iscsi class currently does not support writable sysfs
attrs for LLD sysfs settings. This patch converts the
iscsi class and drivers to use the attribute container
sysfs group and the sysfs group's is_visible callout
to be able to support readable or writable sysfs attrs.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>