NFSD: Add missing NFSv2 .pc_func methods

There's no protection in nfsd_dispatch() against a NULL .pc_func
helpers. A malicious NFS client can trigger a crash by invoking the
unused/unsupported NFSv2 ROOT or WRITECACHE procedures.

The current NFSD dispatcher does not support returning a void reply
to a non-NULL procedure, so the reply to both of these is wrong, for
the moment.

Cc: <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>
Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfsproc.c

···
 	return nfsd_return_attrs(nfserr, resp);
 }
 
+/* Obsolete, replaced by MNTPROC_MNT. */
+static __be32
+nfsd_proc_root(struct svc_rqst *rqstp)
+{
+	return nfs_ok;
+}
+
 /*
  * Look up a path name component
  * Note: the dentry in the resp->fh may be negative if the file
···
 
 	if (nfserr) return nfserr;
 	return fh_getattr(&resp->fh, &resp->stat);
+}
+
+/* Reserved */
+static __be32
+nfsd_proc_writecache(struct svc_rqst *rqstp)
+{
+	return nfs_ok;
 }
 
 /*
···
 		.pc_xdrressize = ST+AT,
 	},
 	[NFSPROC_ROOT] = {
+		.pc_func = nfsd_proc_root,
 		.pc_decode = nfssvc_decode_void,
 		.pc_encode = nfssvc_encode_void,
 		.pc_argsize = sizeof(struct nfsd_void),
···
 		.pc_xdrressize = ST+AT+1+NFSSVC_MAXBLKSIZE_V2/4,
 	},
 	[NFSPROC_WRITECACHE] = {
+		.pc_func = nfsd_proc_writecache,
 		.pc_decode = nfssvc_decode_void,
 		.pc_encode = nfssvc_encode_void,
 		.pc_argsize = sizeof(struct nfsd_void),