ALSA: usb-audio: Fix data race at mixer_ctl_feature_info()

The info callback for USB-audio mixer controls for feature unit has a
dynamic initialization of the contents with the check of
cval->initialized flag.  But, since the info callback may be
concurrently called, this may lead to a data race, giving back an
inconsistent state.  Similarly, get and put callbacks may have
concurrent accesses and can get bogus states.

For avoiding the data race, introduce a mutex locking for the
controls and protect against concurrent info callback calls.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20261007172051.13240-6-tiwai@suse.de

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