t: Fixed exceptional traversal errors mixing up dirty/mutated flags

This function is kinda ugly in that our failed label expects the
dirty/mutated flags to be swapped, but we only swap _after_ calling
lfs3_mtree_traverse to avoid messing up lfs3_mtree_traverse's eot logic.

Long story short, this goto failed after lfs3_mtree_traverse could end
up with drity/mutated in the wrong state.

Worst case, this can leave littlefs in a state where it thinks work was
accomplished, but only if lfs3_mtree_traverse encounters an exceptional
error (LFS3_ERR_IO? LFS3_ERR_CORRUPT?), which usually leads to emergency
actions anyways.

We probably need more testing around exceptional errors like these,
they're also the main limit to our line/branch coverage. But the work
will be tedious so for now that's a future thing.

I at least added a comment to hopefully prevent a similar regression.

Code changes minimal, humorously undoes the LFS3_RDONLY noise:

           code          stack          ctx
  before: 37304           2280          636
  after:  37300 (-0.0%)   2280 (+0.0%)  636 (+0.0%)

lfs3.c

@@ -9927,7 +9927,9 @@ dropped:;
         if (err == LFS3_ERR_NOENT) {
             goto eot;
         }
-        goto failed;
+        // don't goto failed here, we haven't swapped dirty/mutated
+        // flags yet
+        return err;
     }
 
     #ifndef LFS3_RDONLY
@@ -9999,6 +10001,13 @@ dropped:;
     }
     return 0;
 
+    #ifndef LFS3_RDONLY
+failed:;
+    // swap back dirty/mutated flags
+    t->b.o.flags = lfs3_t_swapdirty(t->b.o.flags);
+    return err;
+    #endif
+
 eot:;
     #ifndef LFS3_RDONLY
     // was lookahead scan successful?
@@ -10025,13 +10034,6 @@ eot:;
     #endif
 
     return LFS3_ERR_NOENT;
-
-    #ifndef LFS3_RDONLY
-failed:;
-    // swap back dirty/mutated flags
-    t->b.o.flags = lfs3_t_swapdirty(t->b.o.flags);
-    return err;
-    #endif
 }