nfsd: clear opcnt on compound arg release to prevent OOB read

nfsd4_release_compoundargs() resets args->ops to the inline iops[8]
array when the dynamically-allocated ops buffer is freed, but leaves
args->opcnt at its original value (which can be up to 200 for NFSv4.1+
compounds).

If rq_status_counter is stuck at an odd value (which can happen when
nfsd_dispatch() hits an error path after setting it odd), the RPC
status dumpit handler reads min(opcnt, 16) entries from args->ops[].
Since iops only has 8 elements and is the last field in struct
nfsd4_compoundargs, reading indices 8-15 accesses adjacent slab memory
and leaks it to userspace via netlink.

Zero opcnt unconditionally in nfsd4_release_compoundargs() so stale
compound metadata is never exposed through the status interface.

Fixes: bd9d6a3efa ("NFSD: add rpc_status netlink support")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
[ cel: Remove the kvfree_rcu_mightsleep() sleep from the exposure window ]
Link: https://patch.msgid.link/20260611-nfsd-testing-v2-1-5b90e276f2d9@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

fs/nfsd/nfs4xdr.c

@@ -6434,6 +6434,7 @@ void nfsd4_release_compoundargs(struct svc_rqst *rqstp)
 {
 	struct nfsd4_compoundargs *args = rqstp->rq_argp;
 
+	args->opcnt = 0;
 	if (args->ops != args->iops) {
 		void *old_ops = args->ops;