The only purpose of ibuf_bitmap_mutex is to prevent a deadlock between
two concurrent invocations of ibuf_update_free_bits_for_two_pages_low()
on the same pair of bitmap pages, but in opposite order.
The mutex is unnecessarily serializing the execution of the function
even when it is being invoked on totally different tablespaces.
To avoid deadlocks, it suffices to ensure that the two page latches
are being acquired in a deterministic (sorted) order.