[PATCH] uml: fix x86_64 with !CONFIG_FRAME_POINTER

UML/x86_64 doesn't run when built with frame pointers disabled.  There
was an implicit frame pointer assumption in the stub segfault handler.
With frame pointers disabled, UML dies on handling its first page fault.

The container-of part of this is from Paolo Giarrusso <blaisorblade@yahoo.it>.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>