bitfield.h: add FIELD_MAX() and field_max()

Define FIELD_MAX(), which supplies the maximum value that can be
represented by a field value.  Define field_max() as well, to go
along with the lower-case forms of the field mask functions.

Signed-off-by: Alex Elder <elder@linaro.org>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>