Dropped mode field from lfsr_data_t

Now that in-block fields are limited to 28-bits, we have a few more bits
in our lfsr_data_t size field to encoding things.

This commit uses the top 2-bits to encode one of our 4 different
lfsr_data_t encodings:

- 00--nnnn nnnnnnnn nnnnnnnn nnnnnnnn => buffer poiner
- 01--nnnn nnnnnnnn nnnnnnnn nnnnnnnn => inlined data
- 10--nnnn nnnnnnnn nnnnnnnn nnnnnnnn => on-disk reference
- 11--nnnn nnnnnnnn nnnnnnnn nnnnnnnn => concatenated data pointer
   .--|-------|-------|-------'
   |  |     .-|-------'
   |  |     | '------.
   |  '-----|--------|--------.
   v        v        v        v
  1nnnnnnn 1nnnnnnn 1nnnnnnn 0nnnnnnn <= leb128

Note this still works with a hypothetical 12-bit/10-bit littlefs
variant, where we'd only have 2 spare bits:

- 00nnnnnn nnnnnnnn => buffer poiner
- 01nnnnnn nnnnnnnn => inlined data
- 10nnnnnn nnnnnnnn => on-disk reference
- 11nnnnnn nnnnnnnn => concatenated data pointer
   .|-------'
   |'-------.
   v        v
  1nnnnnnn 0nnnnnnn <= leb128

We don't really care about 8-bit/7-bit, can we even fit an rbyd in a
127-byte block?

The main benefit of this encoding is that lfsr_data_t's pointer fields
get the same space as the two words used to encode on-disk block+off.
This may be useful on systems where ptr=2-word, such as some 16-bit
word/32-bit address devices, and some 2-word CHERI pointer devices.

One interesting thing to note: This encoding is only possible thanks to
the observation that total data size is sufficent information to write
out concatendated datas. We don't really need to know the exact number
until prog time, and during prog we can just iterate over datas until
size is exhausted.

So the size field turns out to be sufficient enough for indicating how
many datas are referenced, saving a data-count field.

Code changes are negligible. It should be noted that _most_ machines
won't benefit from ptr=2-word optimizations, including Thumb, our
benchmark ISA:

           code          stack
  before: 33948           2872
  after:  33912 (-0.1%)   2872 (+0.0%)

lfs.h

@@ -424,18 +424,14 @@ typedef struct lfsr_data {
         } disk;
         struct {
             lfs_ssize_t size;
-            uint8_t mode;
             const uint8_t *buffer;
         } buf;
         struct {
             lfs_ssize_t size;
-            uint8_t mode;
-            uint8_t buf[5];
+            uint8_t buf[8];
         } imm;
         struct {
             lfs_ssize_t size;
-            uint8_t mode;
-            uint8_t count;
             const struct lfsr_data *datas;
         } cat;
     } u;