9p: add missing locking around taking dentry fid list

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

9p: add missing locking around taking dentry fid list

Fix a use-after-free on dentry's d_fsdata fid list when a thread

looks up a fid through dentry while another thread unlinks it:

UAF thread:

refcount_t: addition on 0; use-after-free.

p9_fid_get linux/./include/net/9p/client.h:262

v9fs_fid_find+0x236/0x280 linux/fs/9p/fid.c:129

v9fs_fid_lookup_with_uid linux/fs/9p/fid.c:181

v9fs_fid_lookup+0xbf/0xc20 linux/fs/9p/fid.c:314

v9fs_vfs_getattr_dotl+0xf9/0x360 linux/fs/9p/vfs_inode_dotl.c:400

vfs_statx+0xdd/0x4d0 linux/fs/stat.c:248

Freed by:

p9_fid_destroy (inlined)

p9_client_clunk+0xb0/0xe0 linux/net/9p/client.c:1456

p9_fid_put linux/./include/net/9p/client.h:278

v9fs_dentry_release+0xb5/0x140 linux/fs/9p/vfs_dentry.c:55

v9fs_remove+0x38f/0x620 linux/fs/9p/vfs_inode.c:518

vfs_unlink+0x29a/0x810 linux/fs/namei.c:4335

The problem is that d_fsdata was not accessed under d_lock, because

d_release() normally is only called once the dentry is otherwise no

longer accessible but since we also call it explicitly in v9fs_remove

that lock is required:

move the hlist out of the dentry under lock then unref its fids once

they are no longer accessible.