scsi: target/iscsi: Make iscsit_ta_authentication() respect the output buffer size

[ Upstream commit 35bea5c8 ]

Fixes: e48354ce ("iscsi-target: Add iSCSI fabric support for target v4.1")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Cc: Mike Christie <mchristi@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>