gfs2: Minor run_queue fixes

Provide a better description of why the GLF_DEMOTE_IN_PROGRESS flag
cannot be set.

Function do_xmote() may block, so make sure it isn't called when
nonblock is true.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Andrew Price <anprice@redhat.com>