style: Use static consistently

C doesn't require that we have static on both the declaration and
definition of a function. But stylistically, FreeBSD does. Make that
so. Also make a function static that wasn't because, it seems, that
static and struct look a lot alike in a column of text.

Sponsored by: Netflix
Reviewed by: tsoome, jhibbits
Differential Revision: https://reviews.freebsd.org/D50588