ext2: fix potential use after free

The function ext2_xattr_set calls brelse(bh) to drop the reference count
of bh. After that, bh may be freed. However, following brelse(bh),
it reads bh->b_data via macro HDR(bh). This may result in a
use-after-free bug. This patch moves brelse(bh) after reading field.

CC: stable@vger.kernel.org
Signed-off-by: NPan Bian <bianpan2016@163.com>
Signed-off-by: NJan Kara <jack@suse.cz>

fs/ext2/xattr.c

@@ -612,9 +612,9 @@ bad_block:		ext2_error(sb, "ext2_xattr_set",
 	}
 
 cleanup:
-	brelse(bh);
 	if (!(bh && header == HDR(bh)))
 		kfree(header);
+	brelse(bh);
 	up_write(&EXT2_I(inode)->xattr_sem);
 
 	return error;