btrfs: use the correct superblock to compare fsid in btrfs_validate_super

The function btrfs_validate_super() should verify the fsid in the provided
superblock argument. Because, all its callers expect it to do that.

Such as in the following stack:

   write_all_supers()
       sb = fs_info->super_for_commit;
       btrfs_validate_write_super(.., sb)
         btrfs_validate_super(.., sb, ..)

   scrub_one_super()
	btrfs_validate_super(.., sb, ..)

And
   check_dev_super()
	btrfs_validate_super(.., sb, ..)

However, it currently verifies the fs_info::super_copy::fsid instead,
which is not correct.  Fix this using the correct fsid in the superblock
argument.

CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Tested-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: asj

fs/btrfs/disk-io.c

@@ -2384,11 +2384,10 @@ int btrfs_validate_super(struct btrfs_fs_info *fs_info,
 		ret = -EINVAL;
 	}
 
-	if (memcmp(fs_info->fs_devices->fsid, fs_info->super_copy->fsid,
-		   BTRFS_FSID_SIZE)) {
+	if (memcmp(fs_info->fs_devices->fsid, sb->fsid, BTRFS_FSID_SIZE) != 0) {
 		btrfs_err(fs_info,
 		"superblock fsid doesn't match fsid of fs_devices: %pU != %pU",
-			fs_info->super_copy->fsid, fs_info->fs_devices->fsid);
+			  sb->fsid, fs_info->fs_devices->fsid);
 		ret = -EINVAL;
 	}