ALSA: compress_offload: Use guard() for locking

We can simplify the code gracefully with new guard() macro and co for
automatic cleanup of locks.

The explicit mutex_lock/unlock are still seen only in
snd_compress_wait_for_drain() which does temporary unlock/relocking.

Only the code refactoring, and no functional changes.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20240227085306.9764-3-tiwai@suse.de