kernfs: also call kernfs_set_rev() for positive dentry

kernfs: also call kernfs_set_rev() for positive dentry

A KMSAN warning is reported by Alexander Potapenko:

BUG: KMSAN: uninit-value in kernfs_dop_revalidate+0x61f/0x840
fs/kernfs/dir.c:1053
 kernfs_dop_revalidate+0x61f/0x840 fs/kernfs/dir.c:1053
 d_revalidate fs/namei.c:854
 lookup_dcache fs/namei.c:1522
 __lookup_hash+0x3a6/0x590 fs/namei.c:1543
 filename_create+0x312/0x7c0 fs/namei.c:3657
 do_mkdirat+0x103/0x930 fs/namei.c:3900
 __do_sys_mkdir fs/namei.c:3931
 __se_sys_mkdir fs/namei.c:3929
 __x64_sys_mkdir+0xda/0x120 fs/namei.c:3929
 do_syscall_x64 arch/x86/entry/common.c:51

It seems a positive dentry in kernfs becomes a negative dentry directly
through d_delete() in vfs_rmdir(). dentry->d_time is uninitialized
when accessing it in kernfs_dop_revalidate(), because it is only
initialized when created as negative dentry in kernfs_iop_lookup().

The problem can be reproduced by the following command:

  cd /sys/fs/cgroup/pids && mkdir hi && stat hi && rmdir hi && stat hi

A simple fixes seems to be initializing d->d_time for positive dentry
in kernfs_iop_lookup() as well. The downside is the negative dentry
will be revalidated again after it becomes negative in d_delete(),
because the revison of its parent must have been increased due to
its removal.

Alternative solution is implement .d_iput for kernfs, and assign d_time
for the newly-generated negative dentry in it. But we may need to
take kernfs_rwsem to protect again the concurrent kernfs_link_sibling()
on the parent directory, it is a little over-killing. Now the simple
fix is chosen.

Link: https://marc.info/?l=linux-fsdevel&m=163249838610499
Fixes: c7e7c04274b1 ("kernfs: use VFS negative dentry caching")
Reported-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20210928140750.1274441-1-houtao1@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>