cifs: fix use-after-free on the link name

xfstests generic/011 reported use-after-free bug as follows:

  BUG: KASAN: use-after-free in __d_alloc+0x269/0x859
  Read of size 15 at addr ffff8880078933a0 by task dirstress/952

  CPU: 1 PID: 952 Comm: dirstress Not tainted 6.1.0-rc3+ #77
  Call Trace:
   __dump_stack+0x23/0x29
   dump_stack_lvl+0x51/0x73
   print_address_description+0x67/0x27f
   print_report+0x3e/0x5c
   kasan_report+0x7b/0xa8
   kasan_check_range+0x1b2/0x1c1
   memcpy+0x22/0x5d
   __d_alloc+0x269/0x859
   d_alloc+0x45/0x20c
   d_alloc_parallel+0xb2/0x8b2
   lookup_open+0x3b8/0x9f9
   open_last_lookups+0x63d/0xc26
   path_openat+0x11a/0x261
   do_filp_open+0xcc/0x168
   do_sys_openat2+0x13b/0x3f7
   do_sys_open+0x10f/0x146
   __se_sys_creat+0x27/0x2e
   __x64_sys_creat+0x55/0x6a
   do_syscall_64+0x40/0x96
   entry_SYSCALL_64_after_hwframe+0x63/0xcd

  Allocated by task 952:
   kasan_save_stack+0x1f/0x42
   kasan_set_track+0x21/0x2a
   kasan_save_alloc_info+0x17/0x1d
   __kasan_kmalloc+0x7e/0...