Add lfs_fs_grow to enable limited resizing of the filesystem

[geky]

Originally proposed by @kaetemi, this API allows you to change the size of the filesystem on disk to take advantage of additional space if it becomes available:

 cfg.block_count = 1024;
 
 int err = lfs_mount(lfs, cfg);
 if (err) {
     return err;
 }
 
 struct lfs_fsstat fsstat;
 err = lfs_fs_stat(lfs, &fsstat);
 if (err) {
     return err;
 }
 printf("block_count: %d\n", fsstat.block_count); // prints 1024
 
 err = lfs_fs_grow(lfs, 2048);
 if (err) {
     return err;
 }
 
 struct lfs_fsstat fsstat;
 err = lfs_fs_stat(lfs, &fsstat);
 if (err) {
     return err;
 }
 printf("block_count: %d\n", fsstat.block_count); // prints 2048

If you want to automatically resize littlefs to the full disk size every mount, this can be done by using these two calls as your "mount" operation:

lfs_size_t required_block_count = DISK_SIZE / BLOCK_SIZE;

// unknown block_count
cfg.block_count = 0;

// mount and resize if disk has grown
int err = lfs_mount(lfs, cfg);
if (err) {
    return err;
}

err = lfs_fs_grow(lfs, required_block_count);
if (err) {
    return err;
}

---

See #702, #279, and #753 for related discussions.

This depends on #866, which makes block_size optional

[geky-bot]

Tests passed ✓, Code: 16810 B (+0.8%), Stack: 1432 B (+0.0%), Structs: 800 B (+1.5%)

	Code	Stack	Structs		Coverage
Default	16810 B (+0.8%)	1432 B (+0.0%)	800 B (+1.5%)	Lines	2346/2522 lines (+0.2%)
Readonly	6130 B (+0.1%)	448 B (+0.0%)	800 B (+1.5%)	Branches	1200/1526 branches (-0.0%)
Threadsafe	17670 B (+0.9%)	1432 B (+0.0%)	808 B (+1.5%)		Benchmarks
Multiversion	16870 B (+0.7%)	1432 B (+0.0%)	804 B (+1.5%)	Readed	29369693876 B (+0.0%)
Migrate	18486 B (+0.7%)	1736 B (+0.0%)	804 B (+1.5%)	Proged	1482874766 B (+0.0%)
Error-asserts	17470 B (+0.9%)	1424 B (+0.0%)	800 B (+1.5%)	Erased	1568888832 B (+0.0%)