syscall: make use of include/linux when generating system constants
syscall: make use of include/linux when generating system constants

On Linux include/net directory is just to help porting applications
from BSDs and files under net keep less information than include/linux.
Making use of files under include/linux instead of include/net prevents
lack of information.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/63930043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang