ipmi,smbios: move smbios_walk_table to smbios.h

This function will be used for exposing DMI info as sysctls in the
smbios module (in an upcoming review).

While here, add __packed to the structs.

Reviewed by: dab
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D29270