[patch] powerpc: native atomic_add_unless

[Paul Mackerras]

Nick Piggin writes:

> atomic_add_unless (atomic_inc_not_zero) is used in several hot paths in the
> vfs and I'm planning some uses in the memory manager, so it should be as
> small and fast as possible.
> 
> Joel had a good suggestion to save a register but all bugs are mine.
> 
> Comments?

The implementation looks OK.  I would be interested to know if this
actually makes any measurable difference though.

Paul.