pf: Workaround set but unused warning.

The RB_NEXT macro does not use its middle argument since commit
5fce408cc44c737267aaaf0dcecd3454ba9089cd in 2004 (which ironically
fixed an "unused parameter" warning by introducing this warning in all
consumers). RB_PREV has also copied this unfortunate behavior of an
unused argument.

This results in 'parent' not being used. To workaround, inline the
value of 'parent' as the second argument to RB_NEXT.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D34833