[SCSI] zfcp: Rename sbal_last.

sbal_last is confusing, as it is not the last one actually used,
but just a limit. sbal_limit is a better name.

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>