nfsd: zero out pointers after putting nfsd_files on COPY setup error

At first, I thought this might be a source of nfsd_file overputs, but
the current callers seem to avoid an extra put when nfsd4_verify_copy
returns an error.

Still, it's "bad form" to leave the pointers filled out when we don't
have a reference to them anymore, and that might lead to bugs later.
Zero them out as a defensive coding measure.
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4proc.c

@@ -1214,8 +1214,10 @@ nfsd4_verify_copy(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	return status;
 out_put_dst:
 	nfsd_file_put(*dst);
+	*dst = NULL;
 out_put_src:
 	nfsd_file_put(*src);
+	*src = NULL;
 	goto out;
 }