Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
in blkcg_unuse_delay, blkcg_set_delay and blkcg_clear_delay and
atomic64_try_cmpxchg in blkcg_scale_delay.  x86 CMPXCHG instruction
returns success in ZF flag, so this change saves a compare after cmpxchg
(and related move instruction in front of cmpxchg).

Also, atomic_try_cmpxchg implicitly assigns old *ptr value to "old" when
cmpxchg fails, enabling further code simplifications.

No functional change intended.
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20220712154455.66868-1-ubizjak@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>