ALSA: control: Hardening for potential Spectre v1

As recently Smatch suggested, a few places in ALSA control core codes
may expand the array directly from the user-space value with
speculation:

  sound/core/control.c:1003 snd_ctl_elem_lock() warn: potential spectre issue 'kctl->vd'
  sound/core/control.c:1031 snd_ctl_elem_unlock() warn: potential spectre issue 'kctl->vd'
  sound/core/control.c:844 snd_ctl_elem_info() warn: potential spectre issue 'kctl->vd'
  sound/core/control.c:891 snd_ctl_elem_read() warn: potential spectre issue 'kctl->vd'
  sound/core/control.c:939 snd_ctl_elem_write() warn: potential spectre issue 'kctl->vd'

Although all these seem doing only the first load without further
reference, we may want to stay in a safer side, so hardening with
array_index_nospec() would still make sense.

In this patch, we put array_index_nospec() to the common
snd_ctl_get_ioff*() helpers instead of each caller.  These helpers are
also referred from some drivers, too, and basically all usages are to
calculate the array index from the user-space value, hence it's better
to cover there.

BugLink: https://marc.info/?l=linux-kernel&m=152411496503418&w=2
Reported-by: Dan Carpenter <[email protected]>
Cc: <[email protected]>
Signed-off-by: Takashi Iwai <[email protected]>

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