[stable33] fix: improve share mount conflict resolution logic

[icewind1991]

This backports the improved mount conflict resolution logic from #57295 but keeps it at mount-time.

The old logic way of checking for conflict was causing a recursive filesystem setup which lead to false positives in the conflict detection.

Fixes #57748

[come-nc]

I think this valid share cache based on an id offset is dangerous and can lead to race conditions.

If 2 request concurrently add shares, one may have ids 1 and 3, and validate both, and the other one 1 and 2, and will skip validation of 2 because the last validated share id is 3.

I would feel a lot better if this cache was a map of validated shares. Maybe that’s not possible for performance reasons but then we need another solution, this is too fragile.

[icewind1991]

I don't think there is a case where getting the shares will return 3 but not 2? with auto-increment that isn't inside a transaction.

Storing the map of validated shares put to much of a load on the cache.

The logic has been working (as far as I know) fine since 32, this just moves it around a bit.

[icewind1991]

Also added a backport of 43a9335 to stop some false positives