add noinline_for_stack

People are adding `noinline' in various places to prevent excess stack
consumption due to gcc inlining.  But once this is done, it is quite unobvious
why the `noinline' is present in the code.  We can comment each and every
site, or we can use noinline_for_stack.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>