seq_buf: fix a misleading comment

The comment for seq_buf_has_overflowed() says that an overflow condition is
marked by len == size, but that's not what the code is testing.  Make the
comment match reality.

Link: https://lkml.kernel.org/r/87pm19kp0m.fsf@meer.lwn.net

Fixes: 8cd709ae

 ("tracing: Have seq_buf use full buffer")
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>