[SCSI] zfcp: Rename sbal_curr to sbal_last.

sbal_last is more appropriate, because it matches sbal_first.

Signed-off-by: Martin Peschke <mp3@de.ibm.com>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>