fs: littlefs: Avoid using static buffers that are too small

[pahindman]

The cache_size and lookahead_size are set at compile time using the CONFIG_FS_LITTLEFS_CACHE_SIZE and CONFIG_FS_LITTLEFS_LOOKAHEAD_SIZE values from Kconfig. Those values are also used to statically allocate buffers that are pointed at by the read_buffer, prog_buffer, and lookahead_buffer members of the lfs_config structure.

At runtime, when using a block device, the cache_size and lookahead_size are updated to be multiples of the block device's block_size, which may make them bigger than the original size used to allocate the static buffers. Detect when the size increases and set the corresponding buffer pointers to NULL in those cases. The littlefs module (in lfs_init) handles the buffer pointers being NULL by dynamically allocating appropriately sized buffers (based on the relevant updated size value).

Fixes: #77917

[de-nordic]

Issues:

• in line 837 there is statement that here we are setting "validated/defaulted" values, which means that this change should not be happening in this block
• this is hidden logic that fixes problem by leading us into next, where mounting of LFS takes away heap, and make LFS or other subsystems fail, depending on the order of execution.

I think the proper solution here would be to fail initialization each time when condition happens, with LOG, because in that case we should have stable path of execution that uniformly warns user on misconfiguration.

[pahindman]

Failing with a LOG indicating misconfiguration makes sense to me, thanks! I'll make that change and update the PR after I try it out on my setup.

[pahindman]

I changed it to use LOG_ERR and return when this situation is encountered. Example output with logging enabled:

$ fs mount littlefs /stuff
Error mounting as littlefs: -12
[00:00:15.760,000] <inf> littlefs: LittleFS version 2.9, disk version 2.1
[00:00:15.940,000] <inf> littlefs: FS at SDMMC: is 524288 0x200-byte blocks with 512 cycle
[00:00:15.940,000] <err> littlefs: Configured cache size is too small: 64 < 512
[00:00:15.940,000] <err> littlefs: Configured lookahead size is too small: 32 < 2048
[00:00:15.940,000] <err> fs: fs mount error (-12)

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