ext4: don't count external journal blocks as overhead

This was fixed for ext3 with:

e6d8fb34 ext3: Count internal journal as bsddf overhead in ext3_statfs

but was never fixed for ext4.

With a large external journal and no used disk blocks, df comes
out negative without this, as journal blocks are added to the
overhead & subtracted from used blocks unconditionally.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -3331,8 +3331,8 @@ int ext4_calculate_overhead(struct super_block *sb)
 			memset(buf, 0, PAGE_SIZE);
 		cond_resched();
 	}
-	/* Add the journal blocks as well */
-	if (sbi->s_journal)
+	/* Add the internal journal blocks as well */
+	if (sbi->s_journal && !sbi->journal_bdev)
 		overhead += EXT4_NUM_B2C(sbi, sbi->s_journal->j_maxlen);
 
 	sbi->s_overhead = overhead;