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:           [[ https://bugs.freebsd.org/182448 | 182448 ]]
MFC after:    1 week
Sponsored by: Sandvine Inc