rx: Statically check rx_statisticsAtomic size

rx: Statically check rx_statisticsAtomic size

Currently, rx_GetStatistics assumes that struct rx_statistics and
rx_statisticsAtomic have the same size (we just memcpy between them).
However, this is never checked, and rx_statistics contains many 'int'
fields where rx_statisticsAtomic has rx_atomic_t fields.

If these are not the same size, our rx stats will silently break, so
add a static assert to make sure they are the same size.

Change-Id: I889867f4a85530c30dd15d32d1822144ea128a95
Reviewed-on: https://gerrit.openafs.org/13414
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>