[SCSI] target: Fix memory leak on error path in pscsi_alloc_task

If allocation of pt->pscsi_cdb fails, we need to free the just-allocated
pt or else it will be leaked.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>