tracing: Use seq_buf_used() in seq_buf_to_user() instead of len

commit 5ac4837841 ("tracing: Use trace_seq_used() and seq_buf_used()
instead of len") changed the tracing code to use trace_seq_used() and
seq_buf_used() instead of using the seq_buf len directly to avoid
overflow issues, but missed a spot in seq_buf_to_user() that makes use
of s->len.

Cleaned up the code a bit as well per suggestion of Steve Rostedt.

Link: http://lkml.kernel.org/r/1447703848-2951-1-git-send-email-jsnitsel@redhat.com

Signed-off-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

lib/seq_buf.c

@@ -306,10 +306,12 @@ int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, int cnt)
 	if (!cnt)
 		return 0;
 
-	if (s->len <= s->readpos)
+	len = seq_buf_used(s);
+
+	if (len <= s->readpos)
 		return -EBUSY;
 
-	len = seq_buf_used(s) - s->readpos;
+	len -= s->readpos;
 	if (cnt > len)
 		cnt = len;
 	ret = copy_to_user(ubuf, s->buffer + s->readpos, cnt);