[SCSI] libiscsi: avoid unnecessary multiple NULL assignments

In iscsi_free_task, NULL is assigned to task->sc twice: before and
after kfifo_in invocatoin. Allocating and freeing iscsi_task are guarded
with session->lock, so multiple NULL assignments cause no trouble. But
people reading the source code may be confused.

The second NULL assignment comes from commit:

    3e5c28ad0391389959ccae81c938c7533efb3490

It seems that the line after kfifo_in invocation was introduced
accidentally.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>