Now that we aren't so rigid about the length of the key being passed
in, we need to be a bit more rigorous about checking the length of
the actual data against the claimed length (a'la num_subauths field).
Check for the case where userspace sends us a seemingly valid key
with a num_subauths field that goes beyond the end of the array. If
that happens, return -EIO and invalidate the key.
Also change the other places where we check for malformed keys in this
code to invalidate the key as well.