if_vlan: add a prototype for vlan_input_p

Move the definition of vlan_input_p to net/if.c and its prototype to
if_vlan_var.h, to match the other functions exported from if_vlan.
Remove the previous comment which is now outdated.

This is required for if_bridge to use this function.

Reviewed by: zlei, kp, des, kib
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D50567