Extended mount to check all metadata-pairs

The most common issue with using littlefs in mbed-os is when users
change from littlefs->FAT->littlefs (or with MBR or similar). When this
corrupts the superblock, littlefs tries to fall back to the backup
superblock. However, at this point in the time the old superblock may be
very out-of-date and pointing to an incorrect filesystem.

There's no complete solution to a malicious modification of the
filesystem (short of checking all metadata+data, a very expensive
operation), but we can at least expand our validation to all of the
metadata for the filesystem. This at least catches the common issues
with changing between different filesystems.

features/storage/filesystem/littlefs/littlefs/lfs.c

@@ -2206,6 +2206,10 @@ int lfs_mount(lfs_t *lfs, const struct lfs_config *cfg) {
 
         lfs->root[0] = superblock.d.root[0];
         lfs->root[1] = superblock.d.root[1];
+        if (lfs_paircmp(lfs->root, dir.d.tail) != 0) {
+            err = LFS_ERR_CORRUPT;
+            goto cleanup;
+        }
     }
 
     if (err || memcmp(superblock.d.magic, "littlefs", 8) != 0) {
@@ -2223,10 +2227,18 @@ int lfs_mount(lfs_t *lfs, const struct lfs_config *cfg) {
         goto cleanup;
     }
 
+    // verify that no metadata pairs are corrupt
+    while (!lfs_pairisnull(dir.d.tail)) {
+        err = lfs_dir_fetch(lfs, &dir, dir.d.tail);
+        if (err) {
+            goto cleanup;
+        }
+    }
+
+    // succuessfully mounted
     return 0;
 
 cleanup:
-
     lfs_deinit(lfs);
     return err;
 }