powerpc/perf/hv-24x7: Fix off-by-one error in request_buffer check

request_buffer can hold 254 requests, so if it already has that number of
entries we can't add a new one.

Also, define constant to show where the number comes from.

Fixes: e3ee15dc ("powerpc/perf/hv-24x7: Define add_event_to_24x7_request()")
Reviewed-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/perf/hv-24x7.c

@@ -166,6 +166,10 @@ DEFINE_PER_CPU(struct hv_24x7_hw, hv_24x7_hw);
 DEFINE_PER_CPU(char, hv_24x7_reqb[H24x7_DATA_BUFFER_SIZE]) __aligned(4096);
 DEFINE_PER_CPU(char, hv_24x7_resb[H24x7_DATA_BUFFER_SIZE]) __aligned(4096);
 
+#define MAX_NUM_REQUESTS	((H24x7_DATA_BUFFER_SIZE -		       \
+					sizeof(struct hv_24x7_request_buffer)) \
+					/ sizeof(struct hv_24x7_request))
+
 static char *event_name(struct hv_24x7_event_data *ev, int *len)
 {
 	*len = be16_to_cpu(ev->event_name_len) - 2;
@@ -1107,7 +1111,7 @@ static int add_event_to_24x7_request(struct perf_event *event,
 	int i;
 	struct hv_24x7_request *req;
 
-	if (request_buffer->num_requests > 254) {
+	if (request_buffer->num_requests >= MAX_NUM_REQUESTS) {
 		pr_devel("Too many requests for 24x7 HCALL %d\n",
 				request_buffer->num_requests);
 		return -EINVAL;