[SCSI] sg: push file descriptor list locking down to per-device locking

Push file descriptor list locking down to per-device locking. Let sg_index_lock
only protect device lookup.
sdp->detached is also set and checked with this lock held.

Signed-off-by: Vaughan Cao <vaughan.cao@oracle.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>