VFS/LittleFS: Incorrect cache size validation causes error when default cache size /lookaside buffer size usage is expected

[de-nordic]

Describe the bug

Commit 09574e6 has introduced cache and lookaside buffer validation that is supposed to error out in case when cache size is not aligned to device block size.
The problem with validation is that is directly tries to compare LittleFS instance stored sizes which may have value of 0 at this point; the value 0 in instance is possible as LittleFS initialization contains mechanism that defaults the instance value to system wide setting passed as Kconfig value.
The problem here is that during validation, the instance value is cached into test variable and corrected if 0:

zephyr/subsys/fs/littlefs_fs.c

Lines 780 to 790 in 7c4abb1

	lfs_size_t cache_size = lcp->cache_size;
	if (cache_size == 0) {
	cache_size = CONFIG_FS_LITTLEFS_CACHE_SIZE;
	}
	lfs_size_t lookahead_size = lcp->lookahead_size;
	if (lookahead_size == 0) {
	lookahead_size = CONFIG_FS_LITTLEFS_LOOKAHEAD_SIZE;
	}

but check for validity once again tests against not yet modified instance value:

zephyr/subsys/fs/littlefs_fs.c

Lines 849 to 861 in 7c4abb1

	if (lcp->cache_size < new_cache_size) {
	LOG_ERR("Configured cache size is too small: %d < %d", lcp->cache_size,
	new_cache_size);
	return -ENOMEM;
	}
	lcp->cache_size = new_cache_size;
	if (lcp->lookahead_size < new_lookahead_size) {
	LOG_ERR("Configured lookahead size is too small: %d < %d",
	lcp->lookahead_size, new_lookahead_size);
	return -ENOMEM;
	}
	lcp->lookahead_size = new_lookahead_size;

since the new_cache_size is calculated from block_size it will always be > 0; it has already been established that instance value can be 0, to pick default, but it has not yet been corrected, but the cache_size is already corrected in lines 780-790.
So the test here should check if cache_size/lookahead_size are requesting at least what is calculated from block size.

To Reproduce
It is enough to set cache-size parameter in DTS LittleFS instance or set cache_size of littlefs_cfg, while defining LittleFS data for fs_mount, to 0 to immediate see initialization error.

Expected behavior
Error should only appear in case when set cache size, either directly or default, is not enough to cover device block size.

Impact
Default LittleFS cache and lookaside buffer does not work.

Environment (please complete the following information):

• OS: Any
• Toolchain Zephyr sdk 0.16.3
• Commit SHA ae7e852

Additional context
Affects 4.0.0.
No proper tests for default values to detect the problem.