sgi-xpc: implement opencomplete messaging

sgi-xpc has a window of failure where an open message can be sent and a
subsequent data message can get lost.  We have added a new message
(opencomplete) which closes that window.

Signed-off-by: Robin Holt <holt@sgi.com>
Signed-off-by: Dean Nelson <dcn@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>