Fixed test_ck_spam*'s open file bshrub/btree issues

- In lfsr_mtree_traverse, we traverse open file bshrubs/btrees before
  we validate the gcksum, which means bugs/asserts can slip through
  before we have a chance to detect something is wrong.

  To work around this, I've added an explicit mdir cksum check right
  before we start traversing an open mdir's bshrubs/btrees. If an open
  mdir doesn't match the on-disk state, the on-disk state must contain
  an error (or the RAM, but that's a different story and wayyy out of
  scope).

  It might be better to rearrange lfsr_mtree_traverse to check gcksums
  first, but this will require another look at our traversal clobbering
  logic.

- For a similar reason, ckfetches can't detect open bshrub/btree
  corruption as is. As its name suggests, ckfetches only checks fetches,
  so any corruption after we've fetched bshrubs/btrees in lfsr_file_open
  will go undetected.

  Fortunately this just means we need a full ckmeta-scan in
  test_ck_spam* tests that keep open files.

  In real use, full ckmeta-scans should be preferred anyways. Limiting
  these scans to mtreeonly was just an attempt to better stress btree
  ckfetches.

  At least we're still testing ckmeta+mtreeonly+ckfetches in
  test_ck_spam_dir_fuzz and test_ck_spam_file_fuzz.

This gets the test_ck_spam* tests running under all of the current
interesting ck-modes.

Code changes:

           code          stack          ctx
  before: 38560           2640          644
  after   38572 (+0.0%)   2640 (+0.0%)  644 (+0.0%)

lfs.c

@@ -9477,6 +9477,20 @@ static int lfsr_mtree_traverse_(lfs_t *lfs, lfsr_traversal_t *t,
                 continue;
             }
 
+            // hold on, does our mdir cksum match? if not we found some
+            // sort of error
+            if (t->ot->mdir.rbyd.cksum != t->o.o.mdir.rbyd.cksum) {
+                LFS_DEBUG("Found mdir cksum mismatch %"PRId32" "
+                            "0x{%"PRIx32",%"PRIx32"}, "
+                            "cksum %08"PRIx32" (!= %08"PRIx32")",
+                        t->o.o.mdir.mid >> lfs->mdir_bits,
+                        t->o.o.mdir.rbyd.blocks[0],
+                        t->o.o.mdir.rbyd.blocks[1],
+                        t->o.o.mdir.rbyd.cksum,
+                        t->ot->mdir.rbyd.cksum);
+                return LFS_ERR_CORRUPT;
+            }
+
             // start traversing the file
             const lfsr_file_t *file = (const lfsr_file_t*)t->ot;
             t->o.bshrub = file->o.bshrub;