bug of lfs_cache_zero cause corruption

[nstcl]

The lfs_cache_zero will cause corruption if prog_size != read_size.

static inline void lfs_cache_zero(lfs_t *lfs, lfs_cache_t *pcache) {
    // zero to avoid information leak
    memset(pcache->buffer, 0xff, lfs->cfg->prog_size);
    pcache->block = CACHE_BLOCK_MAGIC;
}

The lfs->cfg->prog_size is error here when prog_size != read_size, especially read_size<prog_size. When read_size<prog_size, the memset function will set part of memory that not belong to it to 0xff, this will cause secrious problem.
I have tested the bug on my board( stm32 + freertos 10.0.1 + littlefs 1.6)

[geky]

Good catch!

I had assumed lfs_cache_zero would only be used on prog caches, then used it for read caches. Here's a fix: #91

In the next major version (#85), the cache sizes will be the same and configured by a separate option.

Thanks for opening the issue!