kernfs: fix suspicious RCU usage in kernfs_put()

kernfs: fix suspicious RCU usage in kernfs_put()

Commit 741c10b096bc ("kernfs: Use RCU to access kernfs_node::name.")
converted the WARN_ONCE() in kernfs_put() to read kn->name and
parent->name via rcu_dereference(), but kernfs_put() has callers that
hold neither kernfs_rwsem nor the RCU read lock. The inode eviction
path driven by memory reclaim is one such case:

  kernfs_put+0x53/0x60 fs/kernfs/dir.c:602
  evict+0x3c2/0xad0 fs/inode.c:846
  iput_final fs/inode.c:1966 [inline]
  iput.part.0+0x605/0xf50 fs/inode.c:2015
  iput+0x35/0x40 fs/inode.c:1981
  dentry_unlink_inode+0x2a1/0x490 fs/dcache.c:467
  __dentry_kill+0x1d0/0x600 fs/dcache.c:670
  shrink_dentry_list+0x180/0x5e0 fs/dcache.c:1174
  prune_dcache_sb+0xea/0x150 fs/dcache.c:1256
  super_cache_scan+0x328/0x550 fs/super.c:223
  ...
  kswapd+0x556/0xba0 mm/vmscan.c:7343

lockdep complains with "suspicious RCU usage" whenever the WARN
fires from such a context.

Wrap the rcu_dereference() calls in an RCU read-side critical section.
Gate on the active-ref check so the lock is only taken when the WARN
is about to fire.

Note that this does not address the underlying imbalance in
kn->active that triggers the WARN.

Fixes: 741c10b096bc ("kernfs: Use RCU to access kernfs_node::name.")
Reported-by: syzbot+0dfe499ea713e0a15bec@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=0dfe499ea713e0a15bec
Signed-off-by: Conor Kotwasinski <conorkotwasinski2024@u.northwestern.edu>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://patch.msgid.link/20260416134315.1474726-1-conorkotwasinski2024@u.northwestern.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>