ext4: enable meta_bg only when new desc blocks are needed

This patch addresses an issue observed when resize_inode is disabled
and an online extension of a filesysyem is performed. When a filesystem
is expanded to a size that does not require a addition of a new
descriptor block, the meta_bg feature is being enabled even though no
part of the filesystem uses this layout.

This patch ensures that the meta_bg feature is only enabled if
any of the added block groups utilize meta_bg layout.
Signed-off-by: Srivathsa Dara <srivathsa.d.dara@oracle.com>
Link: https://lore.kernel.org/r/20240227131329.2608466-1-srivathsa.d.dara@oracle.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/resize.c

@@ -2085,7 +2085,7 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t n_blocks_count)
 		}
 	}
 
-	if ((!resize_inode && !meta_bg) || n_blocks_count == o_blocks_count) {
+	if ((!resize_inode && !meta_bg && n_desc_blocks > o_desc_blocks) || n_blocks_count == o_blocks_count) {
 		err = ext4_convert_meta_bg(sb, resize_inode);
 		if (err)
 			goto out;