Calculate bitmask array sizes correctly in the EventHub

We've gotten lucky to date:  the previous calculation of bitmask array
sizes, (maxval+1)/8 only works properly when 'maxval' is one less than
a multiple of 8.  Fortunately, this has either been the case for us,
or there has been sufficient 'unused' space at the end of the defined
max value range that we haven't wound up overreading/overwriting the
allocated buffers.

Change-Id: I563a93a86644ab9f19489565e06c28e06bb53abc