ALSA: seq: Fix race at timer setup and close

ALSA sequencer code has an open race between the timer setup ioctl and
the close of the client.  This was triggered by syzkaller fuzzer, and
a use-after-free was caught there as a result.

This patch papers over it by adding a proper queue->timer_mutex lock
around the timer-related calls in the relevant code path.

b/28695438
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org>
Git-commit: 3567eb6af614dac436c4b16a8d426f9faed639b3
Git-repo: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git


Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Dennis Cagle <d-cagle@codeaurora.org>
(cherry picked from commit 3567eb6af614dac436c4b16a8d426f9faed639b3)
Change-Id: I398dd27dcdd1241917fd6d127b2debffc2afd413