ALSA: mixer: oss: Fix racy access to slots

ALSA: mixer: oss: Fix racy access to slots

The OSS mixer can reassign the mapping slots dynamically via proc
file.  Although the addition and deletion of those slots are protected
by mixer->reg_mutex, the access to slots aren't, hence this may cause
UAF when the slots in use are deleted concurrently.

This patch applies the mixer->reg_mutex in all appropriate code paths
(i.e. the ioctl functions) that may access slots.

Reported-by: [email protected]
Reviewed-by: Jaroslav Kysela <[email protected]>
Cc: <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>