Fix incorrect cache reuse when seeking from end-of-block

[geky]

In v2.5 (#632 specifically), we introduced an optimization to avoid rereading data when seeking inside the file cache. Unfortunately this used a slightly wrong condition to check if the cache was "live", which meant seeks from end-of-blocks could end up with invalid caches and wrong data. Not great.

The problem is the nuance of when a file's cache is "live":

1. The file is marked as LFS_F_READING or LFS_F_WRITING.

   But we can't reuse the cache when writing, so we only care about LFS_F_READING.

2. file->off != lfs->cfg->block_size (end-of-block).

   This is an optimization to avoid eagerly reading blocks we may not actually care about.

We weren't checking for the end-of-block case, which meant if you seeked from the end of a block to a seemingly valid location in the file cache, you could end up with an invalid cache.

Note that end-of-block may not be powers-of-two due to CTZ skip-list pointers.

---

The fix is to check for the end-of-block case in lfs_file_seek. Note this now matches the need-new-block logic in lfs_file_flushedread.

This logic change may also make lfs_file_seek call lfs_file_flush more often, but only in cases where lfs_file_flush is a noop.

I've also extended the test_seek tests to cover a few more boundary-read cases and prevent a regression in the future.

Found by @wjl and @lrodorigo
Related: #1057, #728

[geky]

Clang:

tests/test_seek.toml:234:65: error: adding 'unsigned long' to a string does not append to the string [-Werror,-Wstring-plus-int]
        __PRETTY_ASSERT_MEM_EQ(buffer, "kittycatcatkittycatcat" + ((off+1) % strlen("kittycatcat")), size);
                                       ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I guess this highlights how difficult it is to write warnings that are both useful and not obnoxious.

[geky-bot]

Tests passed ✓, Code: 17136 B (+0.0%), Stack: 1440 B (+0.0%), Structs: 812 B (+0.0%)

	Code	Stack	Structs		Coverage
Default	17136 B (+0.0%)	1440 B (+0.0%)	812 B (+0.0%)	Lines	2399/2571 lines (-0.0%)
Readonly	6262 B (+0.2%)	448 B (+0.0%)	812 B (+0.0%)	Branches	1285/1618 branches (+0.0%)
Threadsafe	17984 B (+0.0%)	1440 B (+0.0%)	820 B (+0.0%)		Benchmarks
Multiversion	17208 B (+0.0%)	1440 B (+0.0%)	816 B (+0.0%)	Readed	29369693876 B (+0.0%)
Migrate	18824 B (+0.0%)	1736 B (+0.0%)	816 B (+0.0%)	Proged	1482874766 B (+0.0%)
Error-asserts	17900 B (+0.0%)	1432 B (+0.0%)	812 B (+0.0%)	Erased	1568888832 B (+0.0%)