ovl: fix leaked dentry

In the Linux kernel, the following vulnerability has been resolved:

ovl: fix leaked dentry

Since commit 6815f479ca90 ("ovl: use only uppermetacopy state in
ovl_lookup()"), overlayfs doesn't put temporary dentry when there is a
metacopy error, which leads to dentry leaks when shutting down the related
superblock:

overlayfs: refusing to follow metacopy origin for (/file0)
...
BUG: Dentry (____ptrval____){i=3f33,n=file3} still in use (1) [unmount of overlay overlay]
...
WARNING: CPU: 1 PID: 432 at umount_check.cold+0x107/0x14d
CPU: 1 PID: 432 Comm: unmount-overlay Not tainted 5.12.0-rc5 #1
...
RIP: 0010:umount_check.cold+0x107/0x14d
...
Call Trace:
d_walk+0x28c/0x950
? dentry_lru_isolate+0x2b0/0x2b0
? __kasan_slab_free+0x12/0x20
do_one_tree+0x33/0x60
shrink_dcache_for_umount+0x78/0x1d0
generic_shutdown_super+0x70/0x440
kill_anon_super+0x3e/0x70
deactivate_locked_super+0xc4/0x160
deactivate_super+0xfa/0x140
cleanup_mnt+0x22e/0x370
__cleanup_mnt+0x1a/0x30
task_work_run+0x139/0x210
do_exit+0xb0c/0x2820
? __kasan_check_read+0x1d/0x30
? find_held_lock+0x35/0x160
? lock_release+0x1b6/0x660
? mm_update_next_owner+0xa20/0xa20
? reacquire_held_locks+0x3f0/0x3f0
? __sanitizer_cov_trace_const_cmp4+0x22/0x30
do_group_exit+0x135/0x380
__do_sys_exit_group.isra.0+0x20/0x20
__x64_sys_exit_group+0x3c/0x50
do_syscall_64+0x45/0x70
entry_SYSCALL_64_after_hwframe+0x44/0xae
...
VFS: Busy inodes after unmount of overlay. Self-destruct in 5 seconds. Have a nice day...

This fix has been tested with a syzkaller reproducer.