target/fcoe: Add tag support to tcm_fc

Add the 'tag' attribute to FC node ACLs. This is not used by kernel code,
but gives userspace a place to store a tag string. This is used to provide
support for initiator groups in targetcli.

Just copy iscsi's implementation for tcm_fc. A few other fabrics that
support acls also need this, to be submitted separately after the holidays.

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