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.