net/if_dl.h: add param names in prototypes

previously these _KERNEL functions were inconsistent, with some having
parameter names and some not. fix this by giving them all names, per
style(9).

Reviewed by: thj, des
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D50219

(cherry picked from commit 8c7d193e3cbe3d1c9a7f7869f1748bebefabaccf)