ALSA: timer: keep a list of open masters for slave lookup

ALSA: timer: keep a list of open masters for slave lookup

snd_timer_check_slave() still walks all registered timers and all open
timer instances to find a matching master for a newly opened slave.

Maintain a global list of open master instances that can accept slave
links and use it for the slave lookup path instead. This keeps the
existing matching semantics while avoiding the nested walk over
snd_timer_list and each timer open_list_head.

The reverse path in snd_timer_check_master() already scans only the
pending slave list, so this makes both lookup paths closer in shape
without changing the master/slave linking logic.

Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260316-alsa-timer-master-list-v1-1-fb95e547110a@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>