Use array_index_nospec() to sanitize i with respect to speculation.

Note that the user doesn't control i directly, but can make it out
of bounds by not finding a threshold in the array.
Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
[add note about user control, as explained by Masashi]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>