net: move CONFIG_NET guard to top Makefile

net: move CONFIG_NET guard to top Makefile

When CONFIG_NET is disabled, nothing under the net/ directory is
compiled. Move the CONFIG_NET guard to the top Makefile so the net/
directory is entirely skipped.

When Kbuild visits net/Makefile, CONFIG_NET is obvioulsy 'y' because
CONFIG_NET is a bool option. Clean up net/Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20210125231659.106201-1-masahiroy@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>