Renamed cache_size -> buffer_size

This makes littlefs's usage of the term "cache" an entirely internal
concept and hopefully avoids some confusion about the usefulness of
throwing RAM > block_size at these buffers.

The term cache isn't entirely inaccurate, these buffers do act as
single-line caches, however more often the term cache is used to
describe multi-line caches. Maybe this will be added in littlefs's
future, but the code-size cost makes this change not worth the overhead
at the moment.

tests/test_truncate.toml

@@ -100,7 +100,7 @@ code = '''
     lfs_file_open(&lfs, &file, "sequence",
             LFS_O_RDWR | LFS_O_CREAT | LFS_O_TRUNC) => 0;
 
-    size = lfs_min(lfs.cfg->cache_size, sizeof(buffer)/2);
+    size = lfs_min(lfs.cfg->buffer_size, sizeof(buffer)/2);
     lfs_size_t qsize = size / 4;
     uint8_t *wb = buffer;
     uint8_t *rb = buffer + size;