pidfs: Fix memory leak in pidfd_info()

pidfs: Fix memory leak in pidfd_info()

After running the program 'ioctl_pidfd03' of Linux Test Project (LTP) or
the program 'pidfd_info_test' in 'tools/testing/selftests/pidfd' of the
kernel source, kmemleak reports the following memory leaks:

  # cat /sys/kernel/debug/kmemleak
  unreferenced object 0xff110020e5988000 (size 8216):
    comm "ioctl_pidfd03", pid 10853, jiffies 4294800031
    hex dump (first 32 bytes):
      02 40 00 00 00 00 00 00 10 00 00 00 00 00 00 00  .@..............
      00 00 00 00 af 01 00 00 80 00 00 00 00 00 00 00  ................
    backtrace (crc 69483047):
      kmem_cache_alloc_node_noprof+0x2fb/0x410
      copy_process+0x178/0x1740
      kernel_clone+0x99/0x3b0
      __do_sys_clone3+0xbe/0x100
      do_syscall_64+0x7b/0x2c0
      entry_SYSCALL_64_after_hwframe+0x76/0x7e
  ...
  unreferenced object 0xff11002097b70000 (size 8216):
  comm "pidfd_info_test", pid 11840, jiffies 4294889165
  hex dump (first 32 bytes):
    06 40 00 00 00 00 00 00 10 00 00 00 00 00 00 00  .@..............
    00 00 00 00 b5 00 00 00 80 00 00 00 00 00 00 00  ................
  backtrace (crc a6286bb7):
    kmem_cache_alloc_node_noprof+0x2fb/0x410
    copy_process+0x178/0x1740
    kernel_clone+0x99/0x3b0
    __do_sys_clone3+0xbe/0x100
    do_syscall_64+0x7b/0x2c0
    entry_SYSCALL_64_after_hwframe+0x76/0x7e
  ...

The leak occurs because pidfd_info() obtains a task_struct via
get_pid_task() but never calls put_task_struct() to drop the reference,
leaving task->usage unbalanced.

Fix the issue by adding '__free(put_task) = NULL' to the local variable
'task', ensuring that put_task_struct() is automatically invoked when
the variable goes out of scope.

Fixes: 7477d7dce48a ("pidfs: allow to retrieve exit information")
Signed-off-by: Adrian Huang (Lenovo) <adrianhuang0701@gmail.com>
Link: https://lore.kernel.org/20250814094453.15232-1-adrianhuang0701@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>