Fix integer truncation in affecting systat -ifstat

The "systat -ifstat" command was using a u_int to store byte counters.
With a 10Gbps or faster interface, this overflows within the default
5 second refresh period.  Switch to using a uint64_t across the board,
which matches the size used for all counters as of r263102.

PR:		182448
MFC after:	1 week
Sponsored by:	Sandvine Inc