Fixed issue with corruption due to different cache sizes

The lfs_cache_zero function that was recently added assumed a single cache
size, which is incorrect. This would cause a buffer overflow if
read_size != prog_size.

Since lfs_cache_zero is only used for scrubbing prog caches, the fix
here is to use lfs_cache_drop instead on read caches. Info in read
caches should never make its way to disk.

Found by nstcl

lfs.c

@@ -1373,7 +1373,10 @@ int lfs_file_opencfg(lfs_t *lfs, lfs_file_t *file,
     }
 
     // zero to avoid information leak
-    lfs_cache_zero(lfs, &file->cache);
+    lfs_cache_drop(lfs, &file->cache);
+    if ((file->flags & 3) != LFS_O_RDONLY) {
+        lfs_cache_zero(lfs, &file->cache);
+    }
 
     // add to list of files
     file->next = lfs->files;
@@ -2055,8 +2058,8 @@ static int lfs_init(lfs_t *lfs, const struct lfs_config *cfg) {
     }
 
     // zero to avoid information leaks
-    lfs_cache_zero(lfs, &lfs->rcache);
     lfs_cache_zero(lfs, &lfs->pcache);
+    lfs_cache_drop(lfs, &lfs->rcache);
 
     // setup lookahead, round down to nearest 32-bits
     LFS_ASSERT(lfs->cfg->lookahead % 32 == 0);