[SCSI] iscsi_transport: Added Ping support

Added ping support for iscsi adapter, application can use this
interface for diagnostic network connection.

Signed-off-by: Vikas Chaudhary <vikas.chaudhary@qlogic.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>