[SCSI] iscsi class: fix vlan configuration

Userspace was sending the priority/id part of the vlan tag
and sysfs was displaying the id in the vlan file. This
renames the vlan sysfs file to vlan_id to reflect that it
was showing the id and to match the vlan_priority file.
This also adds a ISCSI_NET_PARAM_VLAN_TAG iscsi nl command
to relfect that we are sending down the vlan/priority
part of the tag.

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