[SCSI] trivial scsi_execute_async fix

In scsi_execute_async()'s error path, a struct scsi_io_context
allocated with kmem_cache_alloc() is kfree()'d. Obviously
kmem_cache_free() should be used instead.

Signed-off-by: Arne Redlich <arne.redlich@xiranet.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>