ext4: fix overhead calculations in ext4_stats, again

"overhead" was a write-only variable in this function after commit
952fc18e; we set it to 0 for minixdf, or to sbi->s_overhead if !minixdf,
but never read it again after that.

We need to use it, not sbi->s_overhead, when subtracting out overhead
for f_blocks, or we get the wrong answer for minixdf.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -4790,7 +4790,7 @@ static int ext4_statfs(struct dentry *dentry, struct kstatfs *buf)
 
 	buf->f_type = EXT4_SUPER_MAGIC;
 	buf->f_bsize = sb->s_blocksize;
-	buf->f_blocks = ext4_blocks_count(es) - EXT4_C2B(sbi, sbi->s_overhead);
+	buf->f_blocks = ext4_blocks_count(es) - EXT4_C2B(sbi, overhead);
 	bfree = percpu_counter_sum_positive(&sbi->s_freeclusters_counter) -
 		percpu_counter_sum_positive(&sbi->s_dirtyclusters_counter);
 	/* prevent underflow in case that few free space is available */