knfsd: rpcgss: RPC_GSS_PROC_ DESTROY request will get a bad rpc

If I send a RPC_GSS_PROC_DESTROY message to NFSv4 server, it will reply with a
bad rpc reply which lacks an authentication verifier.  Maybe this patch is
needed.

Send/recv packets as following:

send:

RemoteProcedureCall
    xid
    rpcvers = 2
    prog = 100003
    vers = 4
    proc = 0
    cred = AUTH_GSS
        version = 1
        gss_proc = 3 (RPCSEC_GSS_DESTROY)
        service  = 1 (RPC_GSS_SVC_NONE)
    verf = AUTH_GSS
        checksum

reply:

RemoteProcedureReply
    xid
    msg_type
    reply_stat
    accepted_reply
Signed-off-by: NWei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: N"J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: NNeil Brown <neilb@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

net/sunrpc/auth_gss/svcauth_gss.c

@@ -1092,6 +1092,8 @@ svcauth_gss_accept(struct svc_rqst *rqstp, __be32 *authp)
 		}
 		goto complete;
 	case RPC_GSS_PROC_DESTROY:
+		if (gss_write_verf(rqstp, rsci->mechctx, gc->gc_seq))
+			goto auth_err;
 		set_bit(CACHE_NEGATIVE, &rsci->h.flags);
 		if (resv->iov_len + 4 > PAGE_SIZE)
 			goto drop;