Incorrect masks used in mi_bitmap_is_claimedx_across

In `mi_bitmap_is_claimedx_across`, the middle fields and the post field all compare the field contents against `pre_mask`. Compared to the preceding functions, it looks like these should be using `mid_mask` and `post_mask`, respectively.

I can’t say if this has caused any issues; I've just been reading the code to understand how the atomic operations work.

Locate mi_bitmap_is_claimedx_across and compare its mask checks with the preceding bitmap functions. Verify that middle fields are compared with mid_mask and the post field with post_mask; done means the correction is validated by the repository's relevant tests.

Written by the indexing model from the issue text.