Changed unwritable superblock to ENOSPC for consistency

While ECORRUPT is not a wrong error code, it doesn't match other
instances of hitting a corrupt block during write. During writes, if
blocks are detected as corrupt their data is evicted and moved to a new
clean block. This means that at the end of a disk's lifetime, exhaustion
errors will be reported as ENOSPC when littlefs can't find any new block
to store the data.

This has the benefit of matching behaviour when a new file is written
and no more blocks can be found, due to either a small disk or corrupted
blocks on disk. To littlefs it's like the disk shrinks in size over
time.

lfs.c

@@ -1237,7 +1237,7 @@ relocate:
         // can't relocate superblock, filesystem is now frozen
         if (lfs_pair_cmp(oldpair, (const lfs_block_t[2]){0, 1}) == 0) {
             LFS_WARN("Superblock %"PRIu32" has become unwritable", oldpair[1]);
-            return LFS_ERR_CORRUPT;
+            return LFS_ERR_NOSPC;
         }
 
         // relocate half of pair