Fix the access-above-bottom-of-stack crash.

1. Allows to preserve the valueable optimization

2. Works for NMIs

3.  Doesn't care whether or not there are more of the like instances
   where the stack is left empty.

4. Seems to work for me without the crashes:) 

(akpm: this is still under discussion, although I _think_ it's OK.  You might
want to hold off)

Signed-off-by: Stas Sergeev <stsp@aknet.ru> 
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>