Fixed becksums picking up stale bd cache data

This is more a cludge than a real fix, we should probably be handled in
the bd-wrapper layer, but that's all a mess right now and slated for
future work.

The problem is that our "rewinding" of block-level progs for alignment
leaves our bd caches with stale data. This is normally fine, but then we
go read the stale data to calculate our becksum.

Dropping our caches before calculating the becksums fixes this, at least
temporarily.

lfs.c

@@ -10172,6 +10172,12 @@ static int lfsr_ftree_flush(lfs_t *lfs,
             return err;
         }
 
+        // TODO this is a cludge, but right now our bd layer is a mess,
+        // we need caches to be clean so becksum calculation does not pick
+        // up out-of-date pcaches/rcaches
+        lfs_cache_drop(lfs, &lfs->pcache);
+        lfs_cache_drop(lfs, &lfs->rcache);
+
         // prepare our block pointer
         LFS_ASSERT(bptr.cksize > 0);
         LFS_ASSERT(bptr.cksize <= lfs->cfg->block_size);