hfsplus: fix volume corruption issue for generic/101

The xfstests' test-case generic/101 leaves HFS+ volume
in corrupted state:

sudo ./check generic/101
FSTYP -- hfsplus
PLATFORM -- Linux/x86_64 hfsplus-testing-0001 6.17.0-rc1+ #4 SMP PREEMPT_DYNAMIC Wed Oct 1 15:02:44 PDT 2025
MKFS_OPTIONS -- /dev/loop51
MOUNT_OPTIONS -- /dev/loop51 /mnt/scratch

generic/101 _check_generic_filesystem: filesystem on /dev/loop51 is inconsistent
(see XFSTESTS-2/xfstests-dev/results//generic/101.full for details)

Ran: generic/101
Failures: generic/101
Failed 1 of 1 tests

sudo fsck.hfsplus -d /dev/loop51
** /dev/loop51
Using cacheBlockSize=32K cacheTotalBlock=1024 cacheSize=32768K.
Executing fsck_hfs (version 540.1-Linux).
** Checking non-journaled HFS Plus Volume.
The volume name is untitled
** Checking extents overflow file.
** Checking catalog file.
** Checking multi-linked files.
** Checking catalog hierarchy.
** Checking extended attributes file.
** Checking volume bitmap.
** Checking volume information.
Invalid volume free block count
(It should be 2614350 instead of 2614382)
Verify Status: VIStat = 0x8000, ABTStat = 0x0000 EBTStat = 0x0000
CBTStat = 0x0000 CatStat = 0x00000000
** Repairing volume.
** Rechecking volume.
** Checking non-journaled HFS Plus Volume.
The volume name is untitled
** Checking extents overflow file.
** Checking catalog file.
** Checking multi-linked files.
** Checking catalog hierarchy.
** Checking extended attributes file.
** Checking volume bitmap.
** Checking volume information.
** The volume untitled was repaired successfully.

This test executes such steps: "Test that if we truncate a file
to a smaller size, then truncate it to its original size or
a larger size, then fsyncing it and a power failure happens,
the file will have the range [first_truncate_size, last_size[ with
all bytes having a value of 0x00 if we read it the next time
the filesystem is mounted.".

HFS+ keeps volume's free block count in the superblock.
However, hfsplus_file_fsync() doesn't store superblock's
content. As a result, superblock contains not correct
value of free blocks if a power failure happens.

This patch adds functionality of saving superblock's
content during hfsplus_file_fsync() call.

sudo ./check generic/101
FSTYP -- hfsplus
PLATFORM -- Linux/x86_64 hfsplus-testing-0001 6.18.0-rc3+ #96 SMP PREEMPT_DYNAMIC Wed Nov 19 12:47:37 PST 2025
MKFS_OPTIONS -- /dev/loop51
MOUNT_OPTIONS -- /dev/loop51 /mnt/scratch

generic/101 32s ... 30s
Ran: generic/101
Passed all 1 tests

sudo fsck.hfsplus -d /dev/loop51
** /dev/loop51
Using cacheBlockSize=32K cacheTotalBlock=1024 cacheSize=32768K.
Executing fsck_hfs (version 540.1-Linux).
** Checking non-journaled HFS Plus Volume.
The volume name is untitled
** Checking extents overflow file.
** Checking catalog file.
** Checking multi-linked files.
** Checking catalog hierarchy.
** Checking extended attributes file.
** Checking volume bitmap.
** Checking volume information.
** The volume untitled appears to be OK.

Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>
cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
cc: Yangtao Li <frank.li@vivo.com>
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20251119223219.1824434-1-slava@dubeyko.com
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>