Drop the exporting of empty <linux/byteorder/generic.h>

Fix up the contents of <linux/byteorder/> so that it doesn't export a
content-free generic.h to user space.  This involves:

* Removing the __KERNEL__ tests from generic.h and dropping it from
  Kbuild.
* Wrapping the inclusions of generic.h in both big_endian.h and
  little_endian.h in __KERNEL__ tests.
* Shifting big_endian.h and little_endian.h from header-y to
  unifdef-y in Kbuild.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>