[SCSI] scsi_transport_sas: support link error attributes

For now supporting the ->get_linkerrors method is mandatory.  I'll
probably be beaten to implement the .show_foo variables and different
types of attributes soon..

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>