Revert "ext4: fix superblock checksum calculation race"

This reverts commit acaa5326 which can
result in a ext4_superblock_csum_set() trying to sleep while a
spinlock is being held.

For more discussion of this issue, please see:

https://lore.kernel.org/r/000000000000f50cb705b313ed70@google.com

Reported-by: syzbot+7a4ba6a239b91a126c28@syzkaller.appspotmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -289,18 +289,7 @@ void ext4_superblock_csum_set(struct super_block *sb)
 	if (!ext4_has_metadata_csum(sb))
 		return;
 
-	/*
-	 * Locking the superblock prevents the scenario
-	 * where:
-	 *  1) a first thread pauses during checksum calculation.
-	 *  2) a second thread updates the superblock, recalculates
-	 *     the checksum, and updates s_checksum
-	 *  3) the first thread resumes and finishes its checksum calculation
-	 *     and updates s_checksum with a potentially stale or torn value.
-	 */
-	lock_buffer(EXT4_SB(sb)->s_sbh);
 	es->s_checksum = ext4_superblock_csum(sb, es);
-	unlock_buffer(EXT4_SB(sb)->s_sbh);
 }
 
 ext4_fsblk_t ext4_block_bitmap(struct super_block *sb,