nfsd: Remove unnecessary NULL checks

"cb" is never actually NULL in these functions.

On a quick skim of the history, they seem to have been there from the
beginning. I'm not sure if they originally served a purpose.

Reported-by: Jia-Ju Bai <[e-mail]>
Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfs4callback.c

···
 	if (unlikely(status))
 		return status;
 
-	if (cb != NULL) {
-		status = decode_cb_sequence4res(xdr, cb);
-		if (unlikely(status || cb->cb_seq_status))
-			return status;
-	}
+	status = decode_cb_sequence4res(xdr, cb);
+	if (unlikely(status || cb->cb_seq_status))
+		return status;
 
 	return decode_cb_op_status(xdr, OP_CB_RECALL, &cb->cb_status);
 }
···
 	if (unlikely(status))
 		return status;
 
-	if (cb) {
-		status = decode_cb_sequence4res(xdr, cb);
-		if (unlikely(status || cb->cb_seq_status))
-			return status;
-	}
+	status = decode_cb_sequence4res(xdr, cb);
+	if (unlikely(status || cb->cb_seq_status))
+		return status;
+
 	return decode_cb_op_status(xdr, OP_CB_LAYOUTRECALL, &cb->cb_status);
 }
 #endif /* CONFIG_NFSD_PNFS */
···
 	if (unlikely(status))
 		return status;
 
-	if (cb) {
-		status = decode_cb_sequence4res(xdr, cb);
-		if (unlikely(status || cb->cb_seq_status))
-			return status;
-	}
+	status = decode_cb_sequence4res(xdr, cb);
+	if (unlikely(status || cb->cb_seq_status))
+		return status;
+
 	return decode_cb_op_status(xdr, OP_CB_NOTIFY_LOCK, &cb->cb_status);
 }
 
···
 	if (unlikely(status))
 		return status;
 
-	if (cb) {
-		status = decode_cb_sequence4res(xdr, cb);
-		if (unlikely(status || cb->cb_seq_status))
-			return status;
-	}
+	status = decode_cb_sequence4res(xdr, cb);
+	if (unlikely(status || cb->cb_seq_status))
+		return status;
+
 	return decode_cb_op_status(xdr, OP_CB_OFFLOAD, &cb->cb_status);
 }
 /*