Coalesce last data move and command status for read commands.

Make CTL core and block backend set success status before initiating last
data move for read commands.  Make CAM target and iSCSI frontends detect
such condition and send command status together with data.  New I/O flag
allows to skip duplicate status sending on later fe_done() call.

For Fibre Channel this change saves one of three interrupts per read command,
increasing performance from 126K to 160K IOPS.  For iSCSI this change saves
one of three PDUs per read command, increasing performance from 1M to 1.2M
IOPS.

MFC after:	1 month
Sponsored by:	iXsystems, Inc.