bcachefs: six locks: Fix missing barrier on wait->lock_acquired

bcachefs: six locks: Fix missing barrier on wait->lock_acquired

Six locks do lock handoff via the wakeup path: the thread doing the
wakeup also takes the lock on behalf of the waiter, which means the
waiter only has to look at its waitlist entry, and doesn't have to touch
the lock cacheline while another thread is using it.

Linus noticed that this needs a real barrier, which this patch fixes.

Also add a comment for the should_sleep_fn() error path.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: linux-bcachefs@vger.kernel.org
Cc: linux-kernel@vger.kernel.org