ext4: fix resize when resizing within single group

When resizing file system in the way that the new size of the file
system is still in the same group (no new groups are added), then we can
hit a BUG_ON in ext4_alloc_group_tables()

BUG_ON(flex_gd->count == 0 || group_data == NULL);

because flex_gd->count is zero. The reason is the missing check for such
case, so the code always extend the last group fully and then attempt to
add more groups, but at that time n_blocks_count is actually smaller
than o_blocks_count.

It can be easily reproduced like this:

mkfs.ext4 -b 4096 /dev/sda 30M
mount /dev/sda /mnt/test
resize2fs /dev/sda 50M

Fix this by checking whether the resize happens within the singe group
and only add that many blocks into the last group to satisfy user
request. Then o_blocks_count == n_blocks_count and the resize will exit
successfully without and attempt to add more groups into the fs.

Also fix mixing together block number and blocks count which might be
confusing and can easily lead to off-by-one errors (but it is actually
not the case here since the two occurrence of this mix-up will cancel
each other).

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reported-by: Milan Broz <mbroz@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/resize.c

@@ -1582,7 +1582,7 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t n_blocks_count)
 	ext4_fsblk_t o_blocks_count;
 	ext4_group_t o_group;
 	ext4_group_t n_group;
-	ext4_grpblk_t offset;
+	ext4_grpblk_t offset, add;
 	unsigned long n_desc_blocks;
 	unsigned long o_desc_blocks;
 	unsigned long desc_blocks;
@@ -1605,7 +1605,7 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t n_blocks_count)
 		return 0;
 
 	ext4_get_group_no_and_offset(sb, n_blocks_count - 1, &n_group, &offset);
-	ext4_get_group_no_and_offset(sb, o_blocks_count, &o_group, &offset);
+	ext4_get_group_no_and_offset(sb, o_blocks_count - 1, &o_group, &offset);
 
 	n_desc_blocks = (n_group + EXT4_DESC_PER_BLOCK(sb)) /
 			EXT4_DESC_PER_BLOCK(sb);
@@ -1634,10 +1634,12 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t n_blocks_count)
 	}
 	brelse(bh);
 
-	if (offset != 0) {
-		/* extend the last group */
-		ext4_grpblk_t add;
-		add = EXT4_BLOCKS_PER_GROUP(sb) - offset;
+	/* extend the last group */
+	if (n_group == o_group)
+		add = n_blocks_count - o_blocks_count;
+	else
+		add = EXT4_BLOCKS_PER_GROUP(sb) - (offset + 1);
+	if (add > 0) {
 		err = ext4_group_extend_no_check(sb, o_blocks_count, add);
 		if (err)
 			goto out;