This patch tries to address the still remaining issues in ALSA hrtimer
driver:
- Spurious use-after-free was detected in hrtimer callback
- Incorrect rescheduling due to delayed start
- WARN_ON() is triggered in hrtimer_forward() invoked in hrtimer
  callback

The first issue happens only when the new timer is scheduled even
while hrtimer is being closed.  It's related with the second and third
items; since ALSA timer core invokes hw.start callback during hrtimer
interrupt, this may result in the explicit call of hrtimer_start().

Also, the similar problem is seen for the stop; ALSA timer core
invokes hw.stop callback even in the hrtimer handler, too.  Since we
must not call the synced hrtimer_cancel() in such a context, it's just
a hrtimer_try_to_cancel() call that doesn't properly work.

Another culprit of the second and third items is the call of
hrtimer_forward_now() before snd_timer_interrupt().  The timer->stick
value may change during snd_timer_interrupt() call, but this
possibility is ignored completely.

For covering these subtle and messy issues, the following changes have
been done in this patch:
- A new flag, in_callback, is introduced in the private data to
  indicate that the hrtimer handler is being processed.
- Both start and stop callbacks skip when called from (during)
  in_callback flag.
- The hrtimer handler returns properly HRTIMER_RESTART and NORESTART
  depending on the running state now.
- The hrtimer handler reprograms the expiry properly after
  snd_timer_interrupt() call, instead of before.
- The close callback clears running flag and sets in_callback flag
  to block any further start/stop calls.

Signed-off-by: Takashi Iwai <tiwai@suse.de>