[SCSI] sg: completely protect sfds

sfds is protected by sg_index_lock - except for sg_open(), where it
isn't.  Change that and add some documentation.

Signed-off-by: Joern Engel <joern@logfs.org>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>