hfsplus: fix generic/037 xfstests failure

hfsplus: fix generic/037 xfstests failure

The xfstests' test-case generic/037 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/037 - output mismatch (see xfstests-dev/results//generic/037.out.bad)

The goal of generic/037 test-case is to "verify that replacing
a xattr's value is an atomic operation". The test "consists of
removing the old value and then inserting the new value in a btree.
This made readers (getxattr and listxattrs) not getting neither
the old nor the new value during a short time window".

The HFS+ has the issue of executing the xattr replace operation
because __hfsplus_setxattr() method [1] implemented it as not
atomic operation [2]:

	if (hfsplus_attr_exists(inode, name)) {
		if (flags & XATTR_CREATE) {
			pr_err("xattr exists yet\n");
			err = -EOPNOTSUPP;
			goto end_setxattr;
		}
		err = hfsplus_delete_attr(inode, name);
		if (err)
			goto end_setxattr;
		err = hfsplus_create_attr(inode, name, value, size);
		if (err)
			goto end_setxattr;
	}

The main issue of the logic that it implements delete and
create of xattr as independent atomic operations, but the replace
operation at whole is not atomic operation. This patch implements
a new hfsplus_replace_attr() method that makes the xattr replace
operation by atomic one. Also, it reworks hfsplus_create_attr() and
hfsplus_delete_attr() with the goal of reusing the common logic
in hfsplus_replace_attr() method.

sudo ./check generic/037
FSTYP         -- hfsplus
PLATFORM      -- Linux/x86_64 hfsplus-testing-0001 6.19.0-rc1+ #47 SMP PREEMPT_DYNAMIC Thu Jan  8 15:37:20 PST 2026
MKFS_OPTIONS  -- /dev/loop51
MOUNT_OPTIONS -- /dev/loop51 /mnt/scratch

generic/037 37s ...  37s
Ran: generic/037
Passed all 1 tests

[1] https://elixir.bootlin.com/linux/v6.19-rc4/source/fs/hfsplus/xattr.c#L261
[2] https://elixir.bootlin.com/linux/v6.19-rc4/source/fs/hfsplus/xattr.c#L338

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/20260109234213.2805400-1-slava@dubeyko.com
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>