scsi: target: drop unnecessary get_fabric_name() accessor from fabric_ops

All fabrics return a const string. In all cases *except* iSCSI the
get_fabric_name() string matches fabric_ops.name.

Both fabric_ops.get_fabric_name() and fabric_ops.name are user-facing, with
the former being used for PR/ALUA state and the latter for ConfigFS
(config/target/$name), so we unfortunately need to keep both strings around
for now.  Replace the useless .get_fabric_name() accessor function with a
const string fabric_name member variable.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>