perf/core: add perf_get_event() to return perf_event given a struct file

A new extern function, perf_get_event(), is added to return a perf event
given a struct file. This function will be used in later patches.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 67612ce..6eeab86 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -11212,6 +11212,14 @@ struct file *perf_event_get(unsigned int fd)
 	return file;
 }
 
+const struct perf_event *perf_get_event(struct file *file)
+{
+	if (file->f_op != &perf_fops)
+		return ERR_PTR(-EINVAL);
+
+	return file->private_data;
+}
+
 const struct perf_event_attr *perf_event_attrs(struct perf_event *event)
 {
 	if (!event)