[SCSI] sg: remove closed flag

After sg_release() has been called, noone should be able to actually use
that filedescriptor anymore.  So if closed ever made a difference in the
past five years or so, it would have meant a bug.  Remove it.

Signed-off-by: Joern Engel <joern@logfs.org>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
[jejb: fix up checkpatch warnings]
Signed-off-by: James Bottomley <JBottomley@Parallels.com>