tracing: Fix crash from reading trace_pipe with sendfile

tracing: Fix crash from reading trace_pipe with sendfile

Orabug: 23331131

[ Upstream commit a29054d9478d0435ab01b7544da4f674ab13f533 ]

If tracing contains data and the trace_pipe file is read with sendfile(),
then it can trigger a NULL pointer dereference and various BUG_ON within the
VM code.

There's a patch to fix this in the splice_to_pipe() code, but it's also a
good idea to not let that happen from trace_pipe either.

Link: http://lkml.kernel.org/r/1457641146-9068-1-git-send-email-rabin@rab.in
Cc: stable@vger.kernel.org # 2.6.30+
Reported-by: Rabin Vincent <rabin.vincent@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
(cherry picked from commit ab31b690cdf88cf5cb0493718b89dbc37dd784a3)

Signed-off-by: Dan Duval <dan.duval@oracle.com>