[SCSI] sg: fix races with ioctl(SG_IO)

sg_io_owned needs to be set before the command is sent to the midlevel;
otherwise, a quickly-completing command may cause a different CPU
to see "srp->done == 1 && !srp->sg_io_owned", which would lead to
incorrect behavior.

Check srp->done and set srp->orphan while holding rq_list_lock to
prevent races with sg_rq_end_io().

There is no need to check sfp->closed from read/write/ioctl/poll/etc.
since the kernel guarantees that this won't happen.

The usefulness of sg_srp_done() was questionable before; now it is
definitely not needed.

Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>