Cleanup include for files under include/linux

Adding include/linux/anything is bad form. This fixes this prior to
other changes that touches this Makefile.
Makefiles: change EXTRA_CFLAGS to ccflags-y

According to section 3.7 of Documentation/kbuild/makefiles.txt, using
EXTRA_CFLAGS in Makefiles is "still supported but their usage is
deprecated." However, using make EXTRA_CFLAGS="-DSOMETHING" results in
EXTRA_CFLAGS from Makefiles being overwritten, obviously breaking the
build.  This patch converts to them to the newer ccflags-y which also
fixes the problem.