af_unix: Clean up #include under net/unix/.

af_unix: Clean up #include under net/unix/.

net/unix/*.c include many unnecessary header files (rtnetlink.h,
netdevice.h, etc).

Let's clean them up.

af_unix.c:

  +uapi/linux/sockios.h   : Only exist under include/uapi
  +uapi/linux/termios.h   : Only exist under include/uapi

  -linux/freezer.h        : No longer use freezable_schedule_timeout()
  -linux/in.h             : No ipv4_is_XXX() etc
  -linux/module.h         : No longer support CONFIG_UNIX=m
  -linux/netdevice.h      : No dev used
  -linux/rtnetlink.h      : Not part of rtnetlink API
  -linux/signal.h         : signal_pending() is defined in sched/signal.h
  -linux/stat.h           : No struct stat used
  -net/checksum.h         : CHECKSUM_UNNECESSARY is defined in skbuff.h

diag.c:

  +linux/dcache.h         : struct dentry in sk_diag_dump_vfs()
  +linux/user_namespace.h : struct user_namespace in sk_diag_dump_uid()
  +uapi/linux/unix_diag.h : Only exist under include/uapi/

garbage.c:

  +linux/list.h           : struct unix_{vertex,edge}, etc
  +linux/workqueue.h      : DECLARE_WORK(unix_gc_work, ...)

  -linux/file.h           : No fget() etc
  -linux/kernel.h         : No cond_resched() etc
  -linux/netdevice.h      : No dev used
  -linux/proc_fs.h        : No procfs provided
  -linux/string.h         : No memcpy(), kmemdup(), etc

sysctl_net_unix.c:

  +linux/string.h         : kmemdup()
  +net/net_namespace.h    : struct net, net_eq()

  -linux/mm.h             : slab.h is enough

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250318034934.86708-5-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>