scsi: target/core: Use kmem_cache_free() instead of kfree()

memory allocated by kmem_cache_alloc() should be freed using
kmem_cache_free(), not kfree().

Fixes: ad669505 ("scsi: target/core: Make sure that target_wait_for_sess_cmds() waits long enough")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>