ext4: fix e2fsprogs checksum failure for mounted filesystem

Commit 81414b4dd48 ("ext4: remove redundant sb checksum
recomputation") removed checksum recalculation after updating
superblock free space / inode counters in ext4_fill_super() based on
the fact that we will recalculate the checksum on superblock
writeout.

That is correct assumption but until the writeout happens (which can
take a long time) the checksum is incorrect in the buffer cache and if
programs such as tune2fs or resize2fs is called shortly after a file
system is mounted can fail. So return back the checksum recalculation
and add a comment explaining why.

Fixes: 81414b4dd48f ("ext4: remove redundant sb checksum recomputation")
Cc: [e-mail]
Reported-by: Boyang Xue <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>
Signed-off-by: Theodore Ts'o <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

fs/ext4/super.c

···
 		err = percpu_counter_init(&sbi->s_freeinodes_counter, freei,
 					  GFP_KERNEL);
 	}
+	/*
+	 * Update the checksum after updating free space/inode
+	 * counters.  Otherwise the superblock can have an incorrect
+	 * checksum in the buffer cache until it is written out and
+	 * e2fsprogs programs trying to open a file system immediately
+	 * after it is mounted can fail.
+	 */
+	ext4_superblock_csum_set(sb);
 	if (!err)
 		err = percpu_counter_init(&sbi->s_dirs_counter,
 					  ext4_count_dirs(sb), GFP_KERNEL);