ipfw: Fix comment typos in ipfw struct dyn_data

The bcnt_fwd and bcnt_rev fields are the byte counters,
while the pcnt_fwd and pcnt_rev fields are the packet counters.
Fix the comments that were swapped around.

Signed-off-by: Damjan Jovanovic <damjan.jov@gmail.com>
Reviewed by: jlduran,imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1517