[SCSI] avoid useless free_list lock roundtrips

Avoid hitting the host-wide free_list lock unless we need to put a command
back onto the freelist.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>