hfsplus: fix generic/020 xfstests failure

hfsplus: fix generic/020 xfstests failure

The xfstests' test-case generic/020 fails to execute
correctly:

FSTYP -- hfsplus
PLATFORM -- Linux/x86_64 hfsplus-testing-0001 6.15.0-rc4+ #8 SMP PREEMPT_DYNAMIC Thu May 1 16:43:22 PDT 2025
MKFS_OPTIONS -- /dev/loop51
MOUNT_OPTIONS -- /dev/loop51 /mnt/scratch

generic/020 _check_generic_filesystem: filesystem on /dev/loop50 is inconsistent
(see xfstests-dev/results//generic/020.full for details)

    *** add lots of attributes
    *** check
        *** MAX_ATTRS attribute(s)
        +/mnt/test/attribute_12286: Numerical result out of range
        *** -1 attribute(s)
        *** remove lots of attributes
        ...
        (Run 'diff -u /xfstests-dev/tests/generic/020.out /xfstests-dev/results//generic/020.out.bad' to see the entire diff)

The generic/020 creates more than 100 xattrs and gives its
the names user.attribute_<number> (for example, user.attribute_101).
As the next step, listxattr() is called with the goal to check
the correctness of xattrs creation. However, it was issue
in hfsplus_listxattr() logic. This method re-uses
the fd.key->attr.key_name.unicode and strbuf buffers in the loop
without re-initialization. As a result, part of the previous
name could still remain in the buffers. For example,
user.attribute_101 could be processed before user.attribute_54.
The issue resulted in formation the name user.attribute_541
instead of user.attribute_54. This patch adds initialization of
fd.key->attr.key_name.unicode and strbuf buffers before
calling hfs_brec_goto() method that prepare next name in
the buffer.

HFS+ logic supports only inline xattrs. Such extended attributes
can store values not bigger than 3802 bytes [1]. This limitation
requires correction of generic/020 logic. Finally, generic/020
can be executed without any issue:

sudo ./check generic/020
FSTYP         -- hfsplus
PLATFORM      -- Linux/x86_64 hfsplus-testing-0001 6.19.0-rc1+ #44 SMP PREEMPT_DYNAMIC Mon Dec 22 15:39:00 PST 2025
MKFS_OPTIONS  -- /dev/loop51
MOUNT_OPTIONS -- /dev/loop51 /mnt/scratch

generic/020 31s ...  38s
Ran: generic/020
Passed all 1 tests

[1] https://elixir.bootlin.com/linux/v6.19-rc2/source/include/linux/hfs_common.h#L626

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/20251224002810.1137139-1-slava@dubeyko.com
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>