ALSA: control: Add memory consumption limit to user controls

ALSA control interface allows users to add arbitrary control elements
(called "user controls" or "user elements"), and its resource usage is
limited just by the max number of control sets (currently 32).  This
limit, however, is quite loose: each allocation of control set may
have 1028 elements, and each element may have up to 512 bytes (ILP32) or
1024 bytes (LP64) of value data. Moreover, each control set may contain
the enum strings and TLV data, which can be up to 64kB and 128kB,
respectively.  Totally, the whole memory consumption may go over 38MB --
it's quite large, and we'd rather like to reduce the size.

OTOH, there have been other requests even to increase the max number
of user elements; e.g. ALSA firewire stack require the more user
controls, hence we want to raise the bar, too.

For satisfying both requirements, this patch changes the management of
user controls: instead of setting the upper limit of the number of
user controls, we check the actual memory allocation size and set the
upper limit of the total allocation in bytes.  As long as the memory
consumption stays below the limit, more user controls are allowed than
the current limit 32. At the same time, we set the lower limit (8MB)
as default than the current theoretical limit, in order to lower the
risk of DoS.

As a compromise for lowering the default limit, now the actual memory
limit is defined as a module option, 'max_user_ctl_alloc_size', so that
user can increase/decrease the limit if really needed, too.

Link: https://lore.kernel.org/r/s5htur3zl5e.wl-tiwai@suse.de
Co-developed-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Tested-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210408103149.40357-1-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>

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