ALSA: timer: Fix race at concurrent reads

commit 4dff5c7b7093b19c19d3a100f8a3ad87cb7cd9e7 upstream.

snd_timer_user_read() has a potential race among parallel reads, as
qhead and qused are updated outside the critical section due to
copy_to_user() calls.  Move them into the critical section, and also
sanitize the relevant code a bit.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.2: there's no check for tu->connected to fix up]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc sound/core/timer.c
Simple merge