Switched to CRC as seed collection function instead of xor

As noted by gtaska, we are sitting on a better hash-combining function
than xor: CRC. Previous issues with xor were solvable, but relying on
xor for this isn't really worth the risk when we already have a CRC
function readily available.

To quote a study found by gtaska:

https://michiel.buddingh.eu/distribution-of-hash-values

> CRC32 seems to score really well, but its graph is skewed by the results
> of Dataset 5 (binary numbers), which may or may not be too synthetic to
> be considered a fair benchmark. But even if you substract the results
> from that test, it does not fare significantly worse than other,
> cryptographic hash functions.

lfs.c

@@ -872,8 +872,10 @@ static lfs_stag_t lfs_dir_fetchmatch(lfs_t *lfs,
                 ptag ^= (lfs_tag_t)(lfs_tag_chunk(tag) & 1U) << 31;
 
                 // toss our crc into the filesystem seed for
-                // pseudorandom numbers
-                lfs->seed ^= crc;
+                // pseudorandom numbers, note we use another crc here
+                // as a collection function because it is sufficiently
+                // random and convenient
+                lfs->seed = lfs_crc(lfs->seed, &crc, sizeof(crc));
 
                 // update with what's found so far
                 besttag = tempbesttag;