alpha: Improve atomic_add_unless
alpha: Improve atomic_add_unless

Use ll/sc loops instead of C loops around cmpxchg.
Update the atomic64_add_unless block comment to match the code.

Reviewed-and-Tested-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Signed-off-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Signed-off-by: Richard Henderson <aea4fd857e97ee7d33f60175ba9388a955b9d916@twiddle.net>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs