fuse: fix uninit-value in fuse_dentry_revalidate()

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

fuse: fix uninit-value in fuse_dentry_revalidate()

fuse_dentry_revalidate() may be called with a dentry that didn't had
->d_time initialised. The issue was found with KMSAN, where lookup_open()
calls __d_alloc(), followed by d_revalidate(), as shown below:

=====================================================
BUG: KMSAN: uninit-value in fuse_dentry_revalidate+0x150/0x13d0 fs/fuse/dir.c:394
fuse_dentry_revalidate+0x150/0x13d0 fs/fuse/dir.c:394
d_revalidate fs/namei.c:1030 [inline]
lookup_open fs/namei.c:4405 [inline]
open_last_lookups fs/namei.c:4583 [inline]
path_openat+0x1614/0x64c0 fs/namei.c:4827
do_file_open+0x2aa/0x680 fs/namei.c:4859
[...]

Uninit was created at:
slab_post_alloc_hook mm/slub.c:4466 [inline]
slab_alloc_node mm/slub.c:4788 [inline]
kmem_cache_alloc_lru_noprof+0x382/0x1280 mm/slub.c:4807
__d_alloc+0x55/0xa00 fs/dcache.c:1740
d_alloc_parallel+0x99/0x2740 fs/dcache.c:2604
lookup_open fs/namei.c:4398 [inline]
open_last_lookups fs/namei.c:4583 [inline]
path_openat+0x135f/0x64c0 fs/namei.c:4827
do_file_open+0x2aa/0x680 fs/namei.c:4859
[...]
=====================================================