Changed lfs_dir_alloc to maximize block cycles for new metadata pairs

Previously we only bumped the revision count if an eviction would occur
immediately (and possibly corrupt littlefs). This works, but does risk
an unoptimal superblock size if an almost-exhausted superblock was
allocated during lfs_format.

As pointed out by tim-nordell-nimbelink, we can align the revision count
to maximize the number of block cycles without breaking the existing
requirements of increasing revision counts.

As an added benefit, littlefs's wear-leveling should behave more
consistently after this change.

lfs.c

@@ -1375,8 +1375,12 @@ static int lfs_dir_alloc(lfs_t *lfs, lfs_mdir_t *dir) {
         return err;
     }
 
-    // make sure we don't immediately evict
-    dir->rev += dir->rev & 1;
+    // to make sure we don't immediately evict, align the new revision count
+    // to our block_cycles modulus, see lfs_dir_compact for why our modulus
+    // is tweaked this way
+    if (lfs->cfg->block_cycles > 0) {
+        dir->rev = lfs_alignup(dir->rev, ((lfs->cfg->block_cycles+1)|1));
+    }
 
     // set defaults
     dir->off = sizeof(dir->rev);