ring-buffer: Use rb_page_size() instead of open coded head_page size

There's a helper function to get a ring buffer page size (the number
of bytes of data recorded on the page), called rb_page_size().
Use that instead of open coding it.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 35825a8..d8c267e 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3763,7 +3763,7 @@
 	if (rb_per_cpu_empty(cpu_buffer))
 		return NULL;
 
-	if (iter->head >= local_read(&iter->head_page->page->commit)) {
+	if (iter->head >= rb_page_size(iter->head_page)) {
 		rb_inc_iter(iter);
 		goto again;
 	}