Added powerloss testing over lfsr_mkdir, fixed grm bugs

The grm bugs were mostly issues with:

1. Not maintaining the on-disk grm state in RAM (lfs->grm) correctly,
   this needs to be updated correctly after every commit or littlefs
   gets a confused.

2. lfsr_fs_fixgrm got a bit confused when it was missed when changing
   the no-rm encoding from 0 to -2. Added some inline functions to help
   avoid this in the future.

3. Leaking information due to mixing fixed sized and variable sized
   encodings of the grm delta in places. This is a bit tricky to write
   an assert for as we don't parse the full grm when we see a no-rm grm.

tests/t5_dirs.toml

@@ -1079,6 +1079,58 @@ code = '''
     lfsr_unmount(&lfs) => 0;
 '''
 
+[cases.t5_dirs_mkdir_many_pl]
+defines.N = [1, 2, 4, 8, 16, 32, 64, 128, 256, 512]
+reentrant = true
+code = '''
+    // format once per test
+    lfs_t lfs;
+    int err = lfsr_mount(&lfs, cfg);
+    if (err) {
+        lfsr_format(&lfs, cfg) => 0;
+        lfsr_mount(&lfs, cfg) => 0;
+    }
+
+    // make this many directories
+    for (lfs_size_t i = 0; i < N; i++) {
+        char name[256];
+        sprintf(name, "dir%04d", i);
+        int err = lfsr_mkdir(&lfs, name);
+        assert(!err || err == LFS_ERR_EXIST);
+    }
+
+    // check that our mkdir worked
+    for (lfs_size_t i = 0; i < N; i++) {
+        char name[256];
+        sprintf(name, "dir%04d", i);
+        struct lfs_info info;
+        lfsr_stat(&lfs, name, &info) => 0;
+        assert(strcmp(info.name, name) == 0);
+        assert(info.type == LFS_TYPE_DIR);
+    }
+
+    lfsr_dir_t dir;
+    lfsr_dir_open(&lfs, &dir, "/") => 0;
+    struct lfs_info info;
+    lfsr_dir_read(&lfs, &dir, &info) => 0;
+    assert(strcmp(info.name, ".") == 0);
+    assert(info.type == LFS_TYPE_DIR);
+    lfsr_dir_read(&lfs, &dir, &info) => 0;
+    assert(strcmp(info.name, "..") == 0);
+    assert(info.type == LFS_TYPE_DIR);
+    for (lfs_size_t i = 0; i < N; i++) {
+        char name[256];
+        sprintf(name, "dir%04d", i);
+        lfsr_dir_read(&lfs, &dir, &info) => 0;
+        assert(strcmp(info.name, name) == 0);
+        assert(info.type == LFS_TYPE_DIR);
+    }
+    lfsr_dir_read(&lfs, &dir, &info) => LFS_ERR_NOENT;
+
+    lfsr_unmount(&lfs) => 0;
+'''
+
+
 
 
 #[cases.test_dirs_root]