team: better TEAM_OPTION_TYPE_STRING validation

team: better TEAM_OPTION_TYPE_STRING validation

syzbot reported following splat [1]

Make sure user-provided data contains one nul byte.

[1]
 BUG: KMSAN: uninit-value in string_nocheck lib/vsprintf.c:633 [inline]
 BUG: KMSAN: uninit-value in string+0x3ec/0x5f0 lib/vsprintf.c:714
  string_nocheck lib/vsprintf.c:633 [inline]
  string+0x3ec/0x5f0 lib/vsprintf.c:714
  vsnprintf+0xa5d/0x1960 lib/vsprintf.c:2843
  __request_module+0x252/0x9f0 kernel/module/kmod.c:149
  team_mode_get drivers/net/team/team_core.c:480 [inline]
  team_change_mode drivers/net/team/team_core.c:607 [inline]
  team_mode_option_set+0x437/0x970 drivers/net/team/team_core.c:1401
  team_option_set drivers/net/team/team_core.c:375 [inline]
  team_nl_options_set_doit+0x1339/0x1f90 drivers/net/team/team_core.c:2662
  genl_family_rcv_msg_doit net/netlink/genetlink.c:1115 [inline]
  genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
  genl_rcv_msg+0x1214/0x12c0 net/netlink/genetlink.c:1210
  netlink_rcv_skb+0x375/0x650 net/netlink/af_netlink.c:2543
  genl_rcv+0x40/0x60 net/netlink/genetlink.c:1219
  netlink_unicast_kernel net/netlink/af_netlink.c:1322 [inline]
  netlink_unicast+0xf52/0x1260 net/netlink/af_netlink.c:1348
  netlink_sendmsg+0x10da/0x11e0 net/netlink/af_netlink.c:1892
  sock_sendmsg_nosec net/socket.c:718 [inline]
  __sock_sendmsg+0x30f/0x380 net/socket.c:733
  ____sys_sendmsg+0x877/0xb60 net/socket.c:2573
  ___sys_sendmsg+0x28d/0x3c0 net/socket.c:2627
  __sys_sendmsg net/socket.c:2659 [inline]
  __do_sys_sendmsg net/socket.c:2664 [inline]
  __se_sys_sendmsg net/socket.c:2662 [inline]
  __x64_sys_sendmsg+0x212/0x3c0 net/socket.c:2662
  x64_sys_call+0x2ed6/0x3c30 arch/x86/include/generated/asm/syscalls_64.h:47
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0xcd/0x1e0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: 3d249d4ca7d0 ("net: introduce ethernet teaming device")
Reported-by: syzbot+1fcd957a82e3a1baa94d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1fcd957a82e3a1baa94d
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://patch.msgid.link/20250212134928.1541609-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>