Tweaked how mount responds to invalid grm modes

In theory, it should be perfectly fine to read from a filesystem with an
invalid grm mode, so lfsr_data_readgrm has been tweaked to return
LFS_ERR_INVAL in that case.

That being said, we don't actually support read-only mounts, so the end
behavior is still the same, but this lays the groundwork for readonly
mounts in the future.

This also makes it so invalid grm modes result in LFS_ERR_INVAL when
read-only mounts aren't supported, which is what it should be anyways.

lfs.c

@@ -1970,13 +1970,12 @@ static lfsr_data_t lfsr_data_fromgrm(const lfsr_grm_t *grm,
 
     // first encode the number of grms, this can be 0, 1, or 2 and may
     // be extended to a general purpose leb128 type field in the future
-    // encode no-rm as zero-size
-    uint8_t count = lfsr_grm_count(grm);
+    uint8_t mode = lfsr_grm_count(grm);
     lfs_ssize_t d = 0;
-    buffer[d] = count;
+    buffer[d] = mode;
     d += 1;
 
-    for (int i = 0; i < count; i++) {
+    for (uint8_t i = 0; i < mode; i++) {
         lfs_ssize_t d_ = lfs_toleb128(grm->rms[i], &buffer[d], 5);
         LFS_ASSERT(d_ >= 0);
         d += d_;
@@ -1995,16 +1994,20 @@ static int lfsr_data_readgrm(lfs_t *lfs, lfsr_data_t *data,
     grm->rms[0] = -1;
     grm->rms[1] = -1;
 
-    // get the count from the first byte
-    uint8_t count;
-    lfs_ssize_t d = lfsr_data_read(lfs, data, &count, 1);
-    if (d < 0) {
-        return d;
+    // first read the mode field
+    lfs_size_t mode;
+    int err = lfsr_data_readleb128(lfs, data, (int32_t*)&mode);
+    if (err) {
+        return err;
     }
 
-    LFS_ASSERT(count <= 2);
-    for (int i = 0; i < count; i++) {
-        int err = lfsr_data_readleb128(lfs, data, &grm->rms[i]);
+    // unknown mode? return an error, we may be able to mount read-only
+    if (mode > 2) {
+        return LFS_ERR_INVAL;
+    }
+
+    for (uint8_t i = 0; i < mode; i++) {
+        err = lfsr_data_readleb128(lfs, data, &grm->rms[i]);
         if (err) {
             return err;
         }
@@ -7881,6 +7884,7 @@ static int lfsr_mountinited(lfs_t *lfs) {
     int err = lfsr_data_readgrm(lfs, &LFSR_DATA_BUF(lfs->ggrm, LFSR_GRM_DSIZE),
             &lfs->grm);
     if (err) {
+        // TODO switch to read-only?
         return err;
     }