bcachefs: six locks: Simplify optimistic spinning

osq lock maintainers don't want it to be used outside of kernel/locking/
- but, we can do better.

Since we have lock handoff signalled via waitlist entries, there's no
reason for optimistic spinning to have to look at the lock at all -
aside from checking lock-owner; we can just spin looking at our waitlist
entry.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>