[BUG] ext4: generic/764 failed when using fast_commit option (error loading journal)

[Vyacheslav Kovalevsky]

Hello, when testing ext4 with fast_commit enabled (Ubuntu 24.04, Linux 
6.8) I encountered this test failure:

```log
generic/764        [failed, exit status 1]- output mismatch (see 
/root/xfstests-dev/results//generic/764.out.bad)
    --- tests/generic/764.out   2026-09-16 08:04:58.023597440 +0000
    +++ /root/xfstests-dev/results//generic/764.out.bad 2026-09-16 
08:36:05.821198201 +0000
    @@ -1,2 +1,5 @@
     QA output created by 764
    -Silence is golden
    +mount: /mnt/scratch: mount(2) system call failed: Structure needs 
cleaning.
    +       dmesg(1) may have more information after failed mount 
system call.
    +mount -o acl,user_xattr /dev/mapper/flakey-test.764 /mnt/scratch 
failed
    +(see /root/xfstests-dev/results//generic/764.full for details)
    ...
    (Run 'diff -u /root/xfstests-dev/tests/generic/764.out 
/root/xfstests-dev/results//generic/764.out.bad'  to see the entire diff)
check: possible mount failures (see 
/root/xfstests-dev/results//generic/764.mountfail)
```

764.out.bad:

```log
root@ubuntu:~/xfstests-dev# cat ./results/generic/764.out.bad
QA output created by 764
mount: /mnt/scratch: mount(2) system call failed: Structure needs cleaning.
       dmesg(1) may have more information after failed mount system call.
mount -o acl,user_xattr /dev/mapper/flakey-test.764 /mnt/scratch failed
(see /root/xfstests-dev/results//generic/764.full for details)
```

764.mountfail:

```log
[ 1224.887075] run fstests generic/764 at 2026-09-16 08:36:05
[ 1225.042597] EXT4-fs (dm-0): mounted filesystem 
7cfe268c-25e3-4c01-90c7-c933198fe125 r/w with ordered data mode. Quota 
mode: none.
[ 1225.062913] EXT4-fs (dm-0): unmounting filesystem 
7cfe268c-25e3-4c01-90c7-c933198fe125.
[ 1225.078714] JBD2: journal recovery failed
[ 1225.078717] EXT4-fs (dm-0): error loading journal
```

Config info:

```log
FSTYP         -- ext4
PLATFORM      -- Linux/x86_64 ubuntu 6.8.0-139-generic #139-Ubuntu SMP 
PREEMPT_DYNAMIC Sat Aug  1 03:52:05 UTC 2026
MKFS_OPTIONS  -- -F -O fast_commit /dev/loop1
MOUNT_OPTIONS -- -o acl,user_xattr /dev/loop1 /mnt/scratch
```

OS information:

```json
{
        "Hostname" : "ubuntu",
        "StaticHostname" : "ubuntu",
        "PrettyHostname" : null,
        "DefaultHostname" : "localhost",
        "HostnameSource" : "static",
        "IconName" : "computer-vm",
        "Chassis" : "vm",
        "Deployment" : null,
        "Location" : null,
        "KernelName" : "Linux",
        "KernelRelease" : "6.8.0-139-generic",
        "KernelVersion" : "#139-Ubuntu SMP PREEMPT_DYNAMIC Sat Aug  1 
03:52:05 UTC 2026",
        "OperatingSystemPrettyName" : "Ubuntu 24.04.5 LTS",
        "OperatingSystemCPEName" : null,
        "OperatingSystemHomeURL" : "https://www.ubuntu.com/";,
        "HardwareVendor" : "QEMU",
        "HardwareModel" : "Standard PC _Q35 + ICH9, 2009_",
        "HardwareSerial" : null,
        "FirmwareVersion" : "Arch Linux 1.17.0-2-2",
        "FirmwareVendor" : "SeaBIOS",
        "FirmwareDate" : 1396310400000000,
        "MachineID" : "91b0356bbe954341a29ab510f38d09be",
        "BootID" : "c5a11924033a42469f837e676984f813",
        "ProductUUID" : null
}
```

Also tested on Linux 7.2 (upstream) & 6.17.0-29-generic (Ubuntu 25.10).

I reported this issue previously here (did not know about the existing 
test failing):
<https://lore.kernel.org/linux-ext4/CAAJYhwzHRMipEy=mrH_FwELcSCCk9zK7ifzy0cuuWMzX3wmVtg@xxxxxxxxxxxxxx/>.