NFS: Fix an Oops in nfs_d_automount()

When mounting from a NFSv4 referral, path->dentry can end up being a
negative dentry, so derive the struct nfs_server from the dentry
itself instead.

Fixes: 2b0143b5 ("VFS: normal filesystems (and lustre): d_inode() annotations")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/namespace.c

@@ -147,7 +147,7 @@ struct vfsmount *nfs_d_automount(struct path *path)
 	struct nfs_fs_context *ctx;
 	struct fs_context *fc;
 	struct vfsmount *mnt = ERR_PTR(-ENOMEM);
-	struct nfs_server *server = NFS_SERVER(d_inode(path->dentry));
+	struct nfs_server *server = NFS_SB(path->dentry->d_sb);
 	struct nfs_client *client = server->nfs_client;
 	int timeout = READ_ONCE(nfs_mountpoint_expiry_timeout);
 	int ret;