LinuxKPI: add a bitfield.h implementation.
This code was iteratively implemented during the work on various WiFi
drivers -- from individual functions to a macro-created implementations
for the various bit sized needed (and then extended to more for
comepleteness). Some of the bit combinations do not seem to make sense
so are left out.
The __bf_shf(x) was obtained from D26681 .
Requested by: manu 
Reviewed by: hselasky, manu
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26708