ALSA: Add simple refcount helper functions

There are many open-code to manage the same pattern for refcount +
wakeup sync at closing.  Let's provide the common helper functions to
replace the open-code.

- The recount is kept in struct snd_refcount, where it's initialized
by snd_refcount_init().
- The user can simply reference or unreference via snd_refcount_get()
and snd_refcount_put() functions
- The user can wait for the all usages gone by snd_refcount_sync()

Note that here we use atomic_t instead of refcount_t since the current
users allow reusing the refcount after sync again.  The design of
refcount_t prevents exactly this behavior, so it doesn't fit.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260610154538.51076-2-tiwai@suse.de

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