Added allocation randomization for dynamic wear-leveling

This implements the second step of full dynamic wear-leveling, block
allocation randomization. This is the key part the uniformly distributes
wear across the filesystem, even through reboots.

The entropy actually comes from the filesystem itself, by xoring
together all of the CRCs in the metadata-pairs on the filesystem. While
this sounds like a ridiculous operation, it's easy to do when we already
scan the metadata-pairs at mount time.

This gives us a random number we can use for block allocation.
Unfortunately it's not a great general purpose random generator as the
output only changes every filesystem write. Fortunately that's exactly
when we need our allocator.

---

Additionally, the randomization created a mess for the testing
framework. Fortunately, this method of randomization is deterministic.
A very useful property for reproducing bugs.

tests/test_corrupt.sh

@@ -71,24 +71,25 @@ echo "--- Sanity check ---"
 rm -rf blocks
 lfs_mktree
 lfs_chktree
+BLOCKS="$(ls blocks | grep -vw '[01]')"
 
 echo "--- Block corruption ---"
-for i in {2..33}
+for b in $BLOCKS
 do 
     rm -rf blocks
     mkdir blocks
-    ln -s /dev/zero blocks/$(printf '%x' $i)
+    ln -s /dev/zero blocks/$b
     lfs_mktree
     lfs_chktree
 done
 
 echo "--- Block persistance ---"
-for i in {2..33}
+for b in $BLOCKS
 do 
     rm -rf blocks
     mkdir blocks
     lfs_mktree
-    chmod a-w blocks/$(printf '%x' $i) || true
+    chmod a-w blocks/$b
     lfs_mktree
     lfs_chktree
 done
@@ -96,7 +97,7 @@ done
 echo "--- Big region corruption ---"
 rm -rf blocks
 mkdir blocks
-for i in {2..255}
+for i in {2..512}
 do
     ln -s /dev/zero blocks/$(printf '%x' $i)
 done
@@ -106,7 +107,7 @@ lfs_chktree
 echo "--- Alternating corruption ---"
 rm -rf blocks
 mkdir blocks
-for i in {2..511..2}
+for i in {2..1024..2}
 do
     ln -s /dev/zero blocks/$(printf '%x' $i)
 done