Adopted ceiling division in mbits formula

So now:
               (block_size)
  mbits = nlog2(----------) = nlog2(block_size) - 3
               (     8    )

Instead of:

               (     (block_size))
  mbits = nlog2(floor(----------)) = nlog2(block_size & ~0x7) - 3
               (     (     8    ))

This makes the post-log - 3 formula simpler, which we probably want to
prefer as it avoids a division. And ceiling is arguably more intuitive
corner case behavior.

This may seem like a minor detail, but because mbits is purely
block_size derived and not configurable, any quirks here will become
a permanent compatibility requirement.

And hey, it saves a couple bytes (I'm not really sure why, the division
should've been optimized to a shift):

           code          stack          ctx
  before: 35528           2440          636
  after:  35520 (-0.0%)   2440 (+0.0%)  636 (+0.0%)

scripts/dbgmtree.py

@@ -1556,7 +1556,7 @@ class Mtree:
     def mbits_(block_size):
         if isinstance(block_size, Bd):
             block_size = block_size.block_size
-        return mt.ceil(mt.log2(block_size // 8))
+        return mt.ceil(mt.log2(block_size)) - 3
 
     # convenience function for creating mbits-dependent mids
     def mid(self, mbid, mrid=None):