CVE-2025-68751: s390/fpu: Fix false-positive kmsan report in fpu_vstl()
In the Linux kernel, the following vulnerability has been resolved:

s390/fpu: Fix false-positive kmsan report in fpu_vstl()

A false-positive kmsan report is detected when running ping command.

An inline assembly instruction 'vstl' can write varied amount of bytes
depending on value of 'index' argument. If 'index' > 0, 'vstl' writes
at least 2 bytes.

clang generates kmsan write helper call depending on inline assembly
constraints. Constraints are evaluated compile-time, but value of
'index' argument is known only at runtime.

clang currently generates call to __msan_instrument_asm_store with 1 byte
as size. Manually call kmsan function to indicate correct amount of bytes
written and fix false-positive report.

This change fixes following kmsan reports:

[   36.563119] =====================================================
[   36.563594] BUG: KMSAN: uninit-value in virtqueue_add+0x35c6/0x7c70
[   36.563852]  virtqueue_add+0x35c6/0x7c70
[   36.564016]  virtqueue_add_outbuf+0xa0/0xb0
[   36.564266]  start_xmit+0x288c/0x4a20
[   36.564460]  dev_hard_start_xmit+0x302/0x900
[   36.564649]  sch_direct_xmit+0x340/0xea0
[   36.564894]  __dev_queue_xmit+0x2e94/0x59b0
[   36.565058]  neigh_resolve_output+0x936/0xb40
[   36.565278]  __neigh_update+0x2f66/0x3a60
[   36.565499]  neigh_update+0x52/0x60
[   36.565683]  arp_process+0x1588/0x2de0
[   36.565916]  NF_HOOK+0x1da/0x240
[   36.566087]  arp_rcv+0x3e4/0x6e0
[   36.566306]  __netif_receive_skb_list_core+0x1374/0x15a0
[   36.566527]  netif_receive_skb_list_internal+0x1116/0x17d0
[   36.566710]  napi_complete_done+0x376/0x740
[   36.566918]  virtnet_poll+0x1bae/0x2910
[   36.567130]  __napi_poll+0xf4/0x830
[   36.567294]  net_rx_action+0x97c/0x1ed0
[   36.567556]  handle_softirqs+0x306/0xe10
[   36.567731]  irq_exit_rcu+0x14c/0x2e0
[   36.567910]  do_io_irq+0xd4/0x120
[   36.568139]  io_int_handler+0xc2/0xe8
[   36.568299]  arch_cpu_idle+0xb0/0xc0
[   36.568540]  arch_cpu_idle+0x76/0xc0
[   36.568726]  default_idle_call+0x40/0x70
[   36.568953]  do_idle+0x1d6/0x390
[   36.569486]  cpu_startup_entry+0x9a/0xb0
[   36.569745]  rest_init+0x1ea/0x290
[   36.570029]  start_kernel+0x95e/0xb90
[   36.570348]  startup_continue+0x2e/0x40
[   36.570703]
[   36.570798] Uninit was created at:
[   36.571002]  kmem_cache_alloc_node_noprof+0x9e8/0x10e0
[   36.571261]  kmalloc_reserve+0x12a/0x470
[   36.571553]  __alloc_skb+0x310/0x860
[   36.571844]  __ip_append_data+0x483e/0x6a30
[   36.572170]  ip_append_data+0x11c/0x1e0
[   36.572477]  raw_sendmsg+0x1c8c/0x2180
[   36.572818]  inet_sendmsg+0xe6/0x190
[   36.573142]  __sys_sendto+0x55e/0x8e0
[   36.573392]  __s390x_sys_socketcall+0x19ae/0x2ba0
[   36.573571]  __do_syscall+0x12e/0x240
[   36.573823]  system_call+0x6e/0x90
[   36.573976]
[   36.574017] Byte 35 of 98 is uninitialized
[   36.574082] Memory access of size 98 starts at 0000000007aa0012
[   36.574218]
[   36.574325] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Tainted: G    B            N  6.17.0-dirty #16 NONE
[   36.574541] Tainted: [B]=BAD_PAGE, [N]=TEST
[   36.574617] Hardware name: IBM 3931 A01 703 (KVM/Linux)
[   36.574755] =====================================================

[   63.532541] =====================================================
[   63.533639] BUG: KMSAN: uninit-value in virtqueue_add+0x35c6/0x7c70
[   63.533989]  virtqueue_add+0x35c6/0x7c70
[   63.534940]  virtqueue_add_outbuf+0xa0/0xb0
[   63.535861]  start_xmit+0x288c/0x4a20
[   63.536708]  dev_hard_start_xmit+0x302/0x900
[   63.537020]  sch_direct_xmit+0x340/0xea0
[   63.537997]  __dev_queue_xmit+0x2e94/0x59b0
[   63.538819]  neigh_resolve_output+0x936/0xb40
[   63.539793]  ip_finish_output2+0x1ee2/0x2200
[   63.540784]  __ip_finish_output+0x272/0x7a0
[   63.541765]  ip_finish_output+0x4e/0x5e0
[   63.542791]  ip_output+0x166/0x410
[   63.543771]  ip_push_pending_frames+0x1a2/0x470
[   63.544753]  raw_sendmsg+0x1f06/0x2180
[   63.545033]  inet_sendmsg+0xe6/0x190
[   63.546006]  __sys_sendto+0x55e/0x8e0
---truncated---