nfsd: return RESOURCE not GARBAGE_ARGS on too many ops

A client that sends more than a hundred ops in a single compound
currently gets an rpc-level GARBAGE_ARGS error.

It would be more helpful to return NFS4ERR_RESOURCE, since that gives
the client a better idea how to recover (for example by splitting up the
compound into smaller compounds).

This is all a bit academic since we've never actually seen a reason for
clients to send such long compounds, but we may as well fix it.

While we're there, just use NFSD4_MAX_OPS_PER_COMPOUND == 16, the
constant we already use in the 4.1 case, instead of hard-coding 100.
Chances anyone actually uses even 16 ops per compound are small enough
that I think there's a neglible risk or any regression.

This fixes pynfs test COMP6.

Reported-by: "Lu, Xinyu" <luxy.fnst@cn.fujitsu.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index effeeb4..a0bed2b 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -1703,6 +1703,9 @@ nfsd4_proc_compound(struct svc_rqst *rqstp)
 	status = nfserr_minor_vers_mismatch;
 	if (nfsd_minorversion(args->minorversion, NFSD_TEST) <= 0)
 		goto out;
+	status = nfserr_resource;
+	if (args->opcnt > NFSD_MAX_OPS_PER_COMPOUND)
+		goto out;
 
 	status = nfs41_check_op_ordering(args);
 	if (status) {