[SCSI] be2iscsi: Fix for case where task->sc was cleanedup earlier

This patch fixes a bug where the task->sc was cleaned up earlier on a
different thread, possibly abort, and the completion comes later.

This was causing a crash which has been reported in multiple places

Signed-off-by: Jayamohan Kallickal <jayamohan.kallickal@emulex.com>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>