perf: Update the mmap control page on mmap()

Apparently we didn't update the mmap control page right after mmap(),
which leads to surprises when userspace wants to use it.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Cc: Arun Sharma <asharma@fb.com>
Link: http://lkml.kernel.org/n/tip-dcpi7164djsexmx6ya7lilrc@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@elte.hu>

kernel/events/core.c

@@ -3534,6 +3534,8 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 	event->mmap_user = get_current_user();
 	vma->vm_mm->pinned_vm += event->mmap_locked;
 
+	perf_event_update_userpage(event);
+
 unlock:
 	if (!ret)
 		atomic_inc(&event->mmap_count);