linux/bitfield.h: Add primitives for manipulating bitfields both in host- and fixed...

linux/bitfield.h: Add primitives for manipulating bitfields both in host- and fixed-endian

Imports Al Viro's original Linux commit 00b0c9b82663a, which contains
an in depth explanation and two fixes from Johannes Berg:
 e7d4a95da86e0 "bitfield: fix *_encode_bits()",
 37a3862e12382 "bitfield: add u8 helpers".

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
[s.nawrocki: added empty lines between functions and macros]
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
[mb: squash fix including byteorder.h]
Signed-off-by: Matthias Brugger <mbrugger@suse.com>